Usernames on registration which fail policies cause conflicting error messages

[matrixbot]

This issue was originally created by @MTRNord at matrix-org/matrix-authentication-service#3132.

Ideally it only should show the policy error here since the user was not taken in fact. This feels misleading :)

[matrixbot]

This comment was originally posted by @MTRNord at matrix-org/matrix-authentication-service#3132 (comment).

It seems that https://github.com/matrix-org/matrix-authentication-service/blob/d3eb5be63ead3078f71f142785fc3768dcd7bcd8/crates/handlers/src/views/register.rs#L171-L180 is an insufficient check or at least the error returned on the UI isnt granular enough.

Specifically the if case at https://github.com/matrix-org/matrix-authentication-service/blob/d3eb5be63ead3078f71f142785fc3768dcd7bcd8/crates/matrix-synapse/src/lib.rs#L271-L276 probably is the origin of this as it generalizes