Log errors in verifyChallengeInternal()

It is unexpected for verifyChallengeInternal() to fail, since the
Gatekeeper enrollment of the synthetic password (more specifically, the
Gatekeeper password derived from the synthetic password) should always
be valid.  Therefore, ensure that an error message is logged in all
failure cases.

Bug: 282947979
Change-Id: Ic8d0134bc4f3e0979bf500ffb5a99f2522251d39