session-worker: Use clearer message errors on max retries and using auth method infos

Some PAM modules we use by default (such as the fingerprint one) supports this, so in case we receive a max-retries error we should inform the user with some clearer message.

Then, when we use a gdm well known authentication method, we can be a bit more clear when showing the error to the user on what explicitly failed.

So add a function to get the authentication method name and use it when computing the friendly error message.