Return specific error code when email / phone not supported (#17578)

Implements https://github.com/matrix-org/matrix-spec-proposals/pull/4178

If this would need tests, could you give some idea of what tests would
be needed and how best to add them?

### Pull Request Checklist

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before submitting your pull request -->

* [ ] Pull request is based on the develop branch
* [ ] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [ ] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/17578.misc

@@ -0,0 +1 @@
+Return specific error code when adding an email address / phone number to account is not supported (MSC4178).

synapse/api/errors.py

@@ -70,6 +70,7 @@ class Codes(str, Enum):
     THREEPID_NOT_FOUND = "M_THREEPID_NOT_FOUND"
     THREEPID_DENIED = "M_THREEPID_DENIED"
     INVALID_USERNAME = "M_INVALID_USERNAME"
+    THREEPID_MEDIUM_NOT_SUPPORTED = "M_THREEPID_MEDIUM_NOT_SUPPORTED"
     SERVER_NOT_TRUSTED = "M_SERVER_NOT_TRUSTED"
     CONSENT_NOT_GIVEN = "M_CONSENT_NOT_GIVEN"
     CANNOT_LEAVE_SERVER_NOTICE_ROOM = "M_CANNOT_LEAVE_SERVER_NOTICE_ROOM"

synapse/rest/client/account.py

@@ -350,6 +350,7 @@ class EmailThreepidRequestTokenRestServlet(RestServlet):
             raise SynapseError(
                 400,
                 "Adding an email to your account is disabled on this server",
+                Codes.THREEPID_MEDIUM_NOT_SUPPORTED,
             )
 
         body = parse_and_validate_json_object_from_request(
@@ -456,6 +457,7 @@ class MsisdnThreepidRequestTokenRestServlet(RestServlet):
             raise SynapseError(
                 400,
                 "Adding phone numbers to user account is not supported by this homeserver",
+                Codes.THREEPID_MEDIUM_NOT_SUPPORTED,
             )
 
         ret = await self.identity_handler.requestMsisdnToken(
@@ -498,7 +500,9 @@ class AddThreepidEmailSubmitTokenServlet(RestServlet):
                 "Adding emails have been disabled due to lack of an email config"
             )
             raise SynapseError(
-                400, "Adding an email to your account is disabled on this server"
+                400,
+                "Adding an email to your account is disabled on this server",
+                Codes.THREEPID_MEDIUM_NOT_SUPPORTED,
             )
 
         sid = parse_string(request, "sid", required=True)

synapse/util/msisdn.py

@@ -21,7 +21,7 @@
 
 import phonenumbers
 
-from synapse.api.errors import SynapseError
+from synapse.api.errors import Codes, SynapseError
 
 
 def phone_number_to_msisdn(country: str, number: str) -> str:
@@ -45,7 +45,7 @@ def phone_number_to_msisdn(country: str, number: str) -> str:
     try:
         phoneNumber = phonenumbers.parse(number, country)
     except phonenumbers.NumberParseException:
-        raise SynapseError(400, "Unable to parse phone number")
+        raise SynapseError(400, "Unable to parse phone number", Codes.INVALID_PARAM)
     return phonenumbers.format_number(phoneNumber, phonenumbers.PhoneNumberFormat.E164)[
         1:
     ]