git cl tests: improve readability of test helper.
git cl tests: improve readability of test helper.

Shorten lines and remove useless stacktrace to make sure the actually
usefull exception fits on tandrii@'s screen :)

R=aadb4a11584aa9878242ea5d9e4b7e3429654579@chromium.org

Change-Id: I387da6c70f72a122d64f7a044d201d72a858fc3f
Reviewed-on: https://chromium-review.googlesource.com/475811
Reviewed-by: Sergiy Byelozyorov <aadb4a11584aa9878242ea5d9e4b7e3429654579@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools