Don't use SCRAM-specific "e=invalid-proof" on invalid password.

Don't use SCRAM-specific "e=invalid-proof" on invalid password.

Instead, send the same FATAL message as with other password-based
authentication mechanisms. This gives a more user-friendly message:

psql: FATAL:  password authentication failed for user "test"

instead of:

psql: error received from server in SASL exchange: invalid-proof

Even before this patch, the server sent that FATAL message, after the
SCRAM-specific "e=invalid-proof" message. But libpq would stop at the
SCRAM error message, and not process the ErrorResponse that would come
after that. We could've taught libpq to check for an ErrorResponse after
failed authentication, but it's simpler to modify the server to send only
the ErrorResponse. The SCRAM specification allows for aborting the
authentication at any point, using an application-defined error mechanism,
like PostgreSQL's ErrorResponse. Using the e=invalid-proof message is
optional.

Reported by Jeff Janes.

Discussion: https://www.postgresql.org/message-id/CAMkU%3D1w3jQ53M1OeNfN8Cxd9O%2BA_9VONJivTbYoYRRdRsLT6vA@mail.gmail.com