git_config: include project name in missing ref exception

When syncing in parallel, this exception is hard to trace back to
a specific repo as the relevant log line could have been pushed
out by other repos syncing code.

Change-Id: I382efeec7651e85622aa51e351134aef0148267f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233075
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Nasser Grainawi <nasser@codeaurora.org>
Tested-by: Mike Frysinger <vapier@google.com>