bugfix: Make sure SimpleSAML_Session::getSessionFromRequest() always raises an exception when a transient session is used due to a misconfiguration or a temporary failure fetching an existing session.

Transient sessions are just an exceptional event, and they shouldn't be treated as regular sessions. Therefore, if we are trying to get the current session and end up with a transient one, that's because an error occurred and we should raise an exception. Since exceptions due to secure cookies trying to be set via an insecure channel are likely to be misconfigurations, we treat them like that, raising a SimpleSAML\Error\CriticalConfigurationError.

Additionally, we capture exceptions in the SimpleSAML\Logger::flush() method, ensuring the error reported in #413 doesn't happen again.

This resolves #356.