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`mb run` (i.e., `python tools/mb.py run`) can be used to run
tests under swarming directly. However, it turns out that there
were a couple of places where we were swallowing error output,
making it hard to understand why things were failing silently :).

This CL should fix at least some of those cases, and also tweaks
the output a bit to be a bit more readable.

Change-Id: If6b412a9afad472b35220cb23721002aaaadcd32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623745
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662869}