Exceptions have no '.message' field.

Bug: 1135320
Change-Id: I30aba548895f1863c9cf65bf3e81a370405c32fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2451375Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>