download: unify error handling with sub git calls

We gracefully handle cherry-pick errors, but none of the others
which means people get confusing Python tracebacks.  Move the
main logic in a single GitError try block so we can show pretty
error messages for all of them.

Change-Id: I52cdf6468d21a98de7f65b86d5267b3caabd5af8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259854


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>