unlockDialog: only emit 'failed' on reset after failure/cancel

We currently emit "failed" any time the UserVerifier is reset,
and user verification didn't succeed prior.

A more conceptually clear time to emit "failed" would be if
the UserVerifier is reset and user verification failed prior,
and to emit "failed" if the user cancels unlock.

This commit restructures things to do that. Aside from being
more conceptually clear, it also lays the groundwork for us
to be able to reset the unlock screen without failing.

https://bugzilla.gnome.org/show_bug.cgi?id=683437