When submitting a wrong token to validate a phone number the error is not correct

This issue has been migrated from [#8218](https://github.com/matrix-org/synapse/issues/8218).

---

When adding a phone number to an account, the user receive a `submit_url` in the response, and also a text message on the phone with a token composed by 6 digits.

When submitting a wrong token with
```sh
curl -X POST --data $'{"client_secret":"8f4285f7-ace9-4645-b545-e55e0566ab07","sid":"821337013","token":"111111"}' \
-H "Authorization: Bearer MDAxOGxvY2F0aW9uIG1hdHJp_REDACTED" \
'https://matrix-client.matrix.org/_matrix/client/unstable/add_threepid/msisdn/submit_token'
```
The user receive an error 400 with the following body:
```json
{
"errcode": "M_UNKNOWN",
"error": "Error contacting the identity server"
}
```

The `errcode` should be more specific for instance "M_INVALID_TOKEN" (see https://github.com/matrix-org/sydent/issues/292) and the `error` value is really confusing, because there is no identity server at all on the loop.

Start by reproducing the wrong-token request against the `_matrix/client/unstable/add_threepid/msisdn/submit_token` endpoint and trace where its 400 response is generated. Check the surrounding phone-number validation and identity-server error handling; done means an invalid token returns a specific errcode such as `M_INVALID_TOKEN` and an accurate error message.

Written by the indexing model from the issue text.