Currently, errors somewhere deep in a promise chain are not reported as
test failures. We would print the error, but still return with exit
status 0. This CL fixes this to treat any uncaught exception as
failure, as we already do in {Shell::ExecuteString}.

R=yangguo@chromium.org

Change-Id: Idc4e58cc0d45f8fca3f7b2c5070ab68d317fc00b
Reviewed-on: https://chromium-review.googlesource.com/c/1439378Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59165}