An api error when calling drush cvapi should be treated as a drush error

Currently drush output that it gets an error when a drush cvapi call results in an error - but it does not treat it as an error, meaning it does not bubble up.

Adding these lines to our drush will cause much more foam

```

if (!empty($result['is_error'])) {
  drush_set_error('CIVICRM api error', $result['error_message']);
}

```

Mathieu Lutfy Mark Hanna Jamie McClelland ping