register_server: Improve UX with the "hostname already in use" error.

An even better way than the current json error message recommending the
--registration-transfer option is to return an appropriate error code
and have that get picked up by the register_server command.

The register_server command can then display a more comprehensive,
better formatted error message with proper whitespaces and a pointer to
the documentation.

zerver/lib/exceptions.py

@@ -57,6 +57,7 @@ class ErrorCode(Enum):
     CANNOT_DEACTIVATE_GROUP_IN_USE = auto()
     CANNOT_ADMINISTER_CHANNEL = auto()
     REMOTE_SERVER_VERIFICATION_SECRET_NOT_PREPARED = auto()
+    HOSTNAME_ALREADY_IN_USE_BOUNCER_ERROR = auto()
 
 
 class JsonableError(Exception):