stdout and stderr are bytes in Python 3 and must be decoded before
printing the exception.

Bug: 1060409
Change-Id: Ib855f6104919734b7505aa5e978ebe394a1e9db6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2097595Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>