Identity service submitToken endpoints don't describe how errors work

Link to problem area: https://matrix.org/docs/spec/identity_service/r0.3.0#post-matrix-identity-v2-validate-email-submittoken

Issue

https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$WLw58DvFB8aWuF3vyWPXIBY3w_lyRtuwM8uk3-DQkJs?via=matrix.org&via=libera.chat&via=envs.net

Can https://matrix.org/docs/spec/identity_service/r0.3.0#post-matrix-identity-v2-validate-email-submittoken return success in any case, where the return code is not 200?
I.e. the check for success is just to ensure, you didn't get a different page because of some weird routing or so?

empirically it is meant to return a 200 error code with a standard error object: https://github.com/matrix-org/sydent/blob/443a4b26fa6ffe76a05b8fc6ca035536ffe332b2/sydent/http/servlets/emailservlet.py#L176-L199

unless it's an invalid client_secret

$ curl -vs -X POST -H "Authorization: Bearer redacted" -H "Content-Type: application/json" --data '{"client_secret":"redacted","sid":"unknown","token":"wrong"}' https://vector.im/_matrix/identity/v2/validate/email/submitToken
{"success": false, "errcode": "M_INVALID_PARAM", "error": "The token doesn't match"}
..snip..
< HTTP/2 200

Start with the linked Matrix identity service specification section for POST /_matrix/identity/v2/validate/email/submitToken, then compare its documented behavior with sydent/http/servlets/emailservlet.py around lines 176-199. Document whether invalid parameters return HTTP 200 with the standard error object and clarify the exception for an invalid client_secret.