Improve MatrixError when user tries to create an account with digits only

[bmarty]

Could be nice to send errcode M_INVALID_USERNAME, for the moment, the client receives M_UNKNOWN:

{"errcode":"M_UNKNOWN","error":"Numeric user IDs are reserved for guest users."}

Originally posted by @bmarty in #7625 (comment)

[bmarty]

A similar fix has been done here if it helps: https://github.com/matrix-org/synapse/pull/6151/files