Also add a better warning if 'gclient sync -D' fails due to a modified file.
I've tested this code via deleting a DEP and running gclient sync -D.

Bug: 981149
Change-Id: I97035ac238d163ccb1684c3ee423c223ed0f6299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1891830Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Anthony Polito <apolito@google.com>