review: Actually log an internal server error's root cause

We forgot to log the root cause of an internal server error during
the review/approve of a patch set.  Save its backtrace into the log
to help debug any issues.

Change-Id: I9eae7339ac7fa9dc16f3014682ac4e518a5a5349
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed