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
-            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.
+            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