Check for secdiscardable file missing

When trying to unlock a synthetic password protector whose
"secdiscardable" file has gone missing, return an error instead of
throwing a NullPointerException that crashes system_server.

Note, this scenario is not supposed to ever happen, and if it does there
is still no way to unlock the protector.  This change is merely a
robustness improvement to avoid unnecessarily crashing system_server,
not a fix for any actual underlying issue.

Bug: 269567270
Change-Id: If80fef0292d98ac3d53f7c4685afcaf742bbff2e