get-api-crl: error handling
This merge request improves error handling of get-api-crl.
So far this script returns exit code 0 in all cases, thus it's impossible to check if getting CRL succeeded.
This merge request modifies this behaviour, 1 is returned if the CRL file (/tmp/crl.pem) doesn't exists (after the attempt to get it).
This merge request is a needed for turris/updater!172, which tries to fix issue turris/updater#163.