Handle SASL SCRAM server error responses (#3521)

Add proper error handling for SCRAM-SERVER-FINAL-MESSAGE error attribute.
The SCRAM specification allows servers to return error messages via the 'e'
attribute in the server final message. Currently, these errors are ignored
and authentication fails later during signature verification.

Postgres typically doesn't return this error (see [here](https://github.com/postgres/postgres/blob/2047ad068139f0b8c6da73d0b845ca9ba30fb33d/src/backend/libpq/auth-scram.c#L423)
on why), but poolers, or other applications using the postgres protocol might,
and it's part of the SCRAM spec, so it probably makes sense for node-postgres
to handle it.

Aligns behaviour with psql, postgrex, and somewhat with pgJDBC
(pgJDBC in particular is stricter with scram errors).

For reference:

- libpq handling it: https://github.com/postgres/postgres/blob/2047ad068139f0b8c6da73d0b845ca9ba30fb33d/src/interfaces/libpq/fe-auth-scram.c#L708