Fix message for SuperordinateDomainInPendingDeleteException

The StatusProhibitsOperationException superclass constructor just wants a raw message to return to the client, so we should pass it an explanation rather than just the superordinate domain name.  In fact, I think the superordinate domain name isn't really necessary since it should be obvious from the hostname in the create/update request what the superordinate domain would be.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156747185

javatests/google/registry/flows/host/HostCreateFlowTest.java

@@ -184,7 +184,7 @@ public class HostCreateFlowTest extends ResourceFlowTestCase<HostCreateFlow, Hos
     clock.advanceOneMilli();
     thrown.expect(
         SuperordinateDomainInPendingDeleteException.class,
-        "example.tld");
+        "Superordinate domain for this hostname is in pending delete");
     runFlow();
   }