5498b958 runs gsutil using 'os.execv',
which, on Windows, apparently causes it to return before completion.

Also add verbosity to '7z' failures on toolchain downloading.

BUG=chromium:445425
TEST=bot
  - Ran on bot, this fix solves the problem that we were seeing.

R=pgervais@chromium.org, sergeyberezin@chromium.org

Review URL: https://codereview.chromium.org/828463003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293507 0039d316-1c4b-4281-b951-d872f2087c98