Devserver: save traceback in exception file for better debugging.

This CL saves traceback msg into exception file when staging for well
detect where the real error happens.

BUG=chromium:700201
TEST=Ran local devserver & raise error & see whether traceback is
properly raised.

Change-Id: Id71a7e1bf3e59c7ec00505edfabae96f1f501e31
(cherry picked from commit 9671a6369034c37271c353b360a865927dce639d)
Reviewed-on: https://chromium-review.googlesource.com/478050
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
1 file changed