Add specific error code for invalid user names.

synapse/api/errors.py

@@ -42,6 +42,7 @@ class Codes(object):
     EXCLUSIVE = "M_EXCLUSIVE"
     THREEPID_AUTH_FAILED = "M_THREEPID_AUTH_FAILED"
     THREEPID_IN_USE = "THREEPID_IN_USE"
+    INVALID_USER_NAME = "M_INVALID_USER_NAME"
 
 
 class CodeMessageException(RuntimeError):

synapse/handlers/register.py

@@ -53,7 +53,8 @@ class RegistrationHandler(BaseHandler):
             raise SynapseError(
                 400,
                 "User ID must only contain characters which do not"
-                " require URL encoding."
+                " require URL encoding.",
+                Codes.INVALID_USER_NAME
             )
 
         user = UserID(localpart, self.hs.hostname)