project: Handle git sso auth failures as repo exit

If a user is not authenticated, repo continues execution and it will
likely result in more of the same errors being printed. A user is also
likely to SIGTERM the process resulting in more errors.

This change stops repo sync if any of repositories can't be fetched to
Git authentcation using sso helper. We could extend this to all Git
authentication

Change-Id: I9e471e063450c0a51f25a5e7f12a83064dfb170c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/438522
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

error.py

@@ -107,6 +107,10 @@ class GitError(RepoError):
         return self.message
 
 
+class GitAuthError(RepoExitError):
+    """Cannot talk to remote due to auth issue."""
+
+
 class GitcUnsupportedError(RepoExitError):
     """Gitc no longer supported."""