Notify if screenshot fails due to locked device

Currently, if the user tries to take a screenshot while the device
is locked, we fail silently. This change adds an error notification
to tell the user what happened.

Bug: 173815237
Fix: 173815237
Test: manual (tried with user locked and unlocked; only change is
additional notification when locked)

Change-Id: I630d2d948b5e19e6b7e6e93bb965ef153baf13c9