[Provingground] Add check to crosdl.py

If gsutil is not installed, detect that early and bail.

TEST=correctly fails with other commands that are not insatlled
BUG=None

Change-Id: I7aa4e88a585df3b9471b4e96e1df3ce4033e53b9
Reviewed-on: https://chromium-review.googlesource.com/699050
Commit-Ready: Katherine Threlkeld <kathrelkeld@chromium.org>
Tested-by: Katherine Threlkeld <kathrelkeld@chromium.org>
Reviewed-by: Kalin Stoyanov <kalin@chromium.org>
1 file changed