Provide useful error messages on flows run during read-only mode (#1425)

We want to keep the read-only-mode-exception as an unchecked exception,
so we introduce a temporary check in the EppController that provides a
specific error message for this situation (rather than letting it fall
through to the generic "command failed" messaging

docs/flows.md

@@ -27,6 +27,8 @@ An EPP flow that creates a new contact.
     *   Resource with this id already exists.
 *   2306
     *   Declining contact disclosure is disallowed by server policy.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactDeleteFlow
 
@@ -51,6 +53,8 @@ or failure message when the process is complete.
     *   Resource status prohibits this operation.
 *   2305
     *   Resource to be deleted has active incoming references.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactInfoFlow
 
@@ -93,6 +97,8 @@ transfer request, which then becomes effective immediately.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferCancelFlow
 
@@ -116,6 +122,8 @@ request.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferQueryFlow
 
@@ -163,6 +171,8 @@ that window, this flow allows the losing client to reject the transfer request.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactTransferRequestFlow
 
@@ -190,6 +200,8 @@ or rejected, and the gaining registrar can also cancel the transfer request.
     *   Resource with this id does not exist.
 *   2304
     *   Resource status prohibits this operation.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## ContactUpdateFlow
 
@@ -214,6 +226,8 @@ An EPP flow that updates a contact.
 *   2306
     *   Cannot add and remove the same value.
     *   Declining contact disclosure is disallowed by server policy.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainCheckFlow
 
@@ -371,6 +385,8 @@ An EPP flow that creates a new domain resource.
     *   Too many nameservers set on this domain.
     *   Domain labels cannot end with a dash.
     *   Only encoded signed marks are supported.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainDeleteFlow
 
@@ -394,6 +410,8 @@ An EPP flow that deletes a domain.
     *   Resource status prohibits this operation.
 *   2305
     *   Domain to be deleted has subordinate hosts.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainInfoFlow
 
@@ -465,6 +483,8 @@ comes in at the exact millisecond that the domain would have expired.
 *   2306
     *   Periods for domain registrations must be specified in years.
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainRestoreRequestFlow
 
@@ -524,6 +544,8 @@ regardless of what the original expiration time was.
     *   Domain is not eligible for restore.
 *   2306
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferApproveFlow
 
@@ -553,6 +575,8 @@ replaced with new ones with the correct approval time.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferCancelFlow
 
@@ -581,6 +605,8 @@ transfer period passed. In this flow, those speculative objects are deleted.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferQueryFlow
 
@@ -633,6 +659,8 @@ transfer period passed. In this flow, those speculative objects are deleted.
     *   The resource does not have a pending transfer.
 *   2303
     *   Resource with this id does not exist.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainTransferRequestFlow
 
@@ -692,6 +720,8 @@ new ones with the correct approval time).
         EPP extension.
     *   Periods for domain registrations must be specified in years.
     *   The requested fees cannot be provided in the requested currency.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## DomainUpdateFlow
 
@@ -747,6 +777,8 @@ statuses are updated at once.
     *   The secDNS:all element must have value 'true' if present.
     *   Too many DS records set on a domain.
     *   Too many nameservers set on this domain.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HelloFlow
 
@@ -805,6 +837,8 @@ allows creating a host name, and if necessary enqueues tasks to update DNS.
     *   Superordinate domain for this hostname is in pending delete.
 *   2306
     *   Host names must be at least two levels below the registry suffix.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HostDeleteFlow
 
@@ -833,6 +867,8 @@ or failure message when the process is complete.
     *   Resource status prohibits this operation.
 *   2305
     *   Resource to be deleted has active incoming references.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## HostInfoFlow
 
@@ -901,6 +937,8 @@ are enqueued to update DNS accordingly.
 *   2306
     *   Cannot add and remove the same value.
     *   Host names must be at least two levels below the registry suffix.
+*   2400
+    *   Registry is currently undergoing maintenance and is in read-only mode.
 
 ## LoginFlow