sync: invert --force-broken with --fail-fast

People seem to not expect the sync process to halt immediately if an
error is encountered.  It's also basically guaranteed to leave their
tree in an incomplete state.  Lets invert the default behavior so we
attempt to sync (both fetch & checkout) all projects.  If an error is
hit, we still exit(1) and show it at the end.

If people want the sync to abort quickly, they can use the new option
--fail-fast.

Bug: https://crbug.com/gerrit/11293
Change-Id: I49dd6c4dc8fd5cce8aa905ee169ff3cbe230eb3d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/234812
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>