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

[matrixbot]

This issue has been migrated from #8218.

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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

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:

{
  "errcode": "M_UNKNOWN",
  "error": "Error contacting the identity server"
}

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