[telepathy-sasl-signon] Only trigger CredentialsNeedUpdate for specific SASL errors

Unfortunately, telepathy's AuthenticationFailed error arrives for many
normal cases, like a connection reset during authentication. We cannot
invalidate credentials indiscriminately.

Instead, watch for the two specific wocky errors that indicate the
server probably rejected our credentials and signal only for those.
[telepathy-sasl-signon] Don't attempt to create credentials when signon session fails

This is not relevant for our accounts architecture, and calling
request_password is harmful.
Merge pull request #16 from special/master

Reduce situations where CredentialsNeedUpdate is used further