Wrap exception handling around all of loadSession
The user might (probably does) have a session even if we haven't actually tried
to load it yet, so wrap the whole loadSession code in the error handler we were
using for restoring sessions so we gracefully handle exceptions that happen
before trying to restore sessions too.
Remove the catch in MatrixChat that sent you to the login screen. This is
never the right way to handle an error condition: we should only display the
login screen if we successfully determined that the user has no session, or
they explicitly chose to blow their sessions away.
