Improve the error code when trying to register using a name reserved for guests. (#8135)

changelog.d/8135.bugfix

@@ -0,0 +1 @@
+Clarify the error code if a user tries to register with a numeric ID. This bug was introduced in v1.15.0.

synapse/handlers/register.py

@@ -124,7 +124,9 @@ class RegistrationHandler(BaseHandler):
             try:
                 int(localpart)
                 raise SynapseError(
-                    400, "Numeric user IDs are reserved for guest users."
+                    400,
+                    "Numeric user IDs are reserved for guest users.",
+                    errcode=Codes.INVALID_USERNAME,
                 )
             except ValueError:
                 pass