upload: exit non-zero when preupload hooks fail

Bug: https://crbug.com/gerrit/13159
Change-Id: Id140b619242c841223c6bc5d4aa0c37a7ce0219d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/276294
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>