Previously gsutil stderr was getting piped into stdout, which
yields bad results if there are non-fatal warnings in stderr.
Additionally, we should fail outright if gsutil fails (by adding
`check = True`) rather than fail later on when we try to sync to
a manifest that is in fact just a stderr dump.
BUG=none
TEST=manual runs with bad gs urls
Change-Id: Id71791d0c3f180bd0601ef2c783a8e8e4afa8f59
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/321935
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>