Currently, 'bot_update' uses the 'gclient' that is on the system path.
Now, it will use the 'gclient.py' that is in the same 'depot_tools'
checkout as the 'bot_update' recipe module.

Also don't ignore "git_cache" move errors.

BUG=664254,663990,663440
TEST=None

Review-Url: https://codereview.chromium.org/2492963002