init: make bad --repo-rev settings more clear

If the user passes a bad --repo-rev setting in a new checkout, add a
tip to the error message that their option is probably bad instead of
just saying "unable to resolve".

If the user has already initialized a checkout, we'd display a raw
traceback which would confuse them.  Swallow that and also include
the --repo-rev tip.

Bug: https://crbug.com/gerrit/15610
Change-Id: I5d72513c7b37bf9bb5d19862fcdfaf0d1f44e886
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/328820
Reviewed-by: Jack Neus <jackneus@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -618,6 +618,7 @@ def _Init(args, gitc_init=False):
             "REPO_URL set correctly?" % url, file=sys.stderr)
 
   except CloneFailure:
+    print('fatal: double check your --repo-rev setting.', file=sys.stderr)
     if opt.quiet:
       print('fatal: repo init failed; run without --quiet to see why',
             file=sys.stderr)