The depot_tools directory may get into a state in which its no longer a
git repo, in which case we can't update it. On Windows this would cause
the update_depot_tools.bat script to fail with a somewhat confusing
error message, this CL fixes that to inform the user what happened and
what they should do about it.

Bug: 1333997
Change-Id: I2c60fc46f1fae7811ff6610e25380d08010d0597
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3718926Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>