Prevent exceptions in pre-reboot verification from crashing system server

An unhandled exception during pre-reboot verification will cause system
server to crash and restart.

Instead of allowing the exception to crash the system server, we now
catch it and fail the corresponding staged session with appropriate
message.

Bug: 170784748
Test: manual
Test: verified that without this fix, any unhandled exception crashes
the system server.

Change-Id: If100796687906be67bc02ea63a78e1b6e291ce7a