Better structure for error response

& add server not trusted error

api/client-server/registration.yaml

@@ -177,12 +177,15 @@ paths:
             type: object
         400:
           description: |-
-            The request was invalid. An errcode of ``M_THREEPID_IN_USE`` is used to inform the user
-            that the email address is already registered to an account on this server, however,
-            if the home server has the ability to send email, it is recommended that the server
+            Part of the request was invalid. This may include one of the following error codes:
+
+            * ``M_THREEPID_IN_USE`` : The email address is already registered to an account on this server.
+              however, if the home server has the ability to send email, it is recommended that the server
               instead send an email to the user with instructions on how to reset their password.
               This prevents malicious parties from being able to determine if a given email address
               has an account on the Home Server in question.
+            * ``M_SERVER_NOT_TRUSTED`` : The ``id_server`` parameter refers to an ID server
+              that is not trusted by this Home Server.
           examples:
             application/json: |-
               {

specification/client_server_api.rst

@@ -116,6 +116,9 @@ Some requests have unique error codes:
 :``M_THREEPID_IN_USE``:
   Sent when a threepid given to an API cannot be used because the same threepid is already in use.
 
+:``M_SERVER_NOT_TRUSTED``:
+  The client's request used a third party server, eg. ID server, that this server does not trust.
+
 .. _sect:txn_ids:
 
 The client-server API typically uses ``HTTP PUT`` to submit requests with a