This behavior got lost through 0bdd7a8d523e. Specifically, errors during
git clone no longer shut down kas. Fix this by reporting back an error
code from _repo_fetch_async and terminate in repos_fetch if it's
non-zero.
Drop the return value of repos_fetch - no caller expects any.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>