Require that DNS writer be set on Registry entities

We ran into a bunch of prober deployment issues this past week when
attempting to spin up a new cluster because the newly created prober
TLDs had null values for the dnsWriter field. Given that VoidDnsWriter
exists, we can require that dnsWriter always be set, and have people
use that if DNS publishing is not required.

Also cleans up a bunch of related inconsistent exception messages and
tests not verifying said exception messages properly.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154325830

java/google/registry/tools/CreateTldCommand.java

@@ -86,7 +86,7 @@ class CreateTldCommand extends CreateOrUpdateTldCommand {
 
   @Override
   Registry getOldRegistry(String tld) {
-    checkState(!getTlds().contains(tld), "TLD already exists");
+    checkState(!getTlds().contains(tld), "TLD '%s' already exists", tld);
     return null;
   }