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. Upon restart, pre-reboot verification will
be retried and it will cause system server to crash again. Thus creating
a loop.

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 sends
system server into a crash loop

Change-Id: Ie3343681601c072add0b2d50cbaa2e088a27d94b
Merged-In: If100796687906be67bc02ea63a78e1b6e291ce7a
(cherry picked from commit 5a2548589560404f9fce4914216564b0333895d4)