Add alias support to REQ helpers for subject/topic.

The alias support is generic here, and we use it for
migrating subject -> topic in REQ_topic().

zerver/lib/exceptions.py

@@ -39,6 +39,7 @@ class ErrorCode(AbstractEnum):
     CSRF_FAILED = ()
     INVITATION_FAILED = ()
     INVALID_ZULIP_SERVER = ()
+    REQUEST_CONFUSING_VAR = ()
 
 class JsonableError(Exception):
     '''A standardized error format we can turn into a nice JSON HTTP response.