Adds error handling logic for pre-existing tags (which can occur
if "archive" is CTRL-C aborted midway through) and for undeletable
branches (which can happen if they are currently checked out in a
working dir).

Change-Id: I27b6da9f5860c307f49cbeabb1b0ccf9cfb28eb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1930023
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Kevin Marshall <kmarshall@chromium.org>