Fix errorcode for disabled registration (#8867)

The spec says we should return `M_FORBIDDEN` when someone tries to register and
registration is disabled.

changelog.d/8867.bugfix

@@ -0,0 +1 @@
+Fix the error code that is returned when a user tries to register on a homeserver on which new-user registration has been disabled.

synapse/rest/client/v2_alpha/register.py

@@ -451,7 +451,7 @@ async def on_POST(self, request):
 
         # == Normal User Registration == (everyone else)
         if not self._registration_enabled:
-            raise SynapseError(403, "Registration has been disabled")
+            raise SynapseError(403, "Registration has been disabled", Codes.FORBIDDEN)
 
         # For regular registration, convert the provided username to lowercase
         # before attempting to register it. This should mean that people who try

tests/rest/client/v2_alpha/test_register.py

@@ -121,6 +121,7 @@ def test_POST_disabled_registration(self):
 
         self.assertEquals(channel.result["code"], b"403", channel.result)
         self.assertEquals(channel.json_body["error"], "Registration has been disabled")
+        self.assertEquals(channel.json_body["errcode"], "M_FORBIDDEN")
 
     def test_POST_guest_registration(self):
         self.hs.config.macaroon_secret_key = "test"