project: fix http error retry logic

When sync moved to consume clone output, it merged stdout & stderr,
but the retry logic in this function is based on stderr only.  Move
it over to checking stdout.

Change-Id: I71bdc18ed25c978055952721e3a768289d7a3bd2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297902


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>