ScreenShield: try harder to become modal, and catch failures

ScreenShield: try harder to become modal, and catch failures

The screenshield was not checking the return value of pushModal(), meaning
that it believed it was fully locked when it was not. Later, calling
popModal() would fail, causing an exception and blocking the unlock.
Now when we fail we include an explanatory message, pointing the user
to the actual cause of the issue.

https://bugzilla.gnome.org/show_bug.cgi?id=689106