sync: report list of failing git trees

When repo sync fails because some git trees are not in clean state and
as such can not be rebased automatically, it is a pain to figure out
which trees are the culprits.

With this patch the list of offending trees is printed when repo sync
reports checkout errors.

TEST=ran 'repo sync' and observed the proper list of directories show
     up after the final error message

Bug: https://crbug.com/gerrit/11293
Change-Id: Icdf1a03e9014ecb184f331f513cc9a2efc7d11ed
Signed-off-by: Vadim Bendebury <vbendeb@google.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/244053
Reviewed-by: Mike Frysinger <vapier@google.com>