ShellUserVerifier: fix cancellation

Ensure that all async callbacks check and ignore G_IO_ERROR_CANCELLED.
Ensure that all runs of authentication have their own GCancellable, so
that .begin() can be called multiple times on the same user verifier.
Check for fingerprint reader when beginning authentication, and not
when reset by GDM.

https://bugzilla.gnome.org/show_bug.cgi?id=682544

js/ui/unlockDialog.js

@@ -166,6 +166,7 @@ const UnlockDialog = new Lang.Class({
     },
 
     _reset: function() {
+        this._userVerifier.clear();
         this._userVerifier.begin(this._userName, new Batch.Hold());
     },