Revert "pam_fprintd: Just return a PAM_AUTH_ERROR on unknown errors"

This reverts commit ecf6b7c3.

The idea of the commit was to make device failures less fatal to the system. Unfortunately, we can fail quickly in this case, and returning PAM_AUTH_ERR means that the user might run into a retry limit due to this.

Go back to reporting PAM_AUTHINFO_UNAVAIL, it appears as the lesser evil right now. Ideally we want to a way to tell the upper stack to retry authentication whenever there is a good opportunity, but to not consider it as an authentication failure.