Use hook_field_validate() API properly

@Damien Tournoud: awesome! thanks for the patch. :)

... But before reviewing, can you please connect with duellj on this briefly? Sorry. I think he was after the same issue like you for other reasons, but his attempt looks slidely different. I would love to see the best of both in. ;) thx

Link to his issue and patch:
#1451288: Link widget should use hook_field_widget_error()

ok after chatting with Damien on IRC he gave me the helpful hint, that actually both are needed. So I'll do it, as he recommended to me to do it: First I will commit this patch here and then I will probably add the other patch manually on top of this.

The 7 year old patch to link.module does not apply to the latest link 7.x-1.x-dev and if still applicable needs a reroll.

Checking patch link.module...
error: while searching for:
  $optional_field_found = FALSE;
  if ($instance['settings']['validate_url'] !== 0 || is_null($instance['settings']['validate_url']) || !isset($instance['settings']['validate_url'])) {
    foreach ($items as $delta => $value) {
      _link_validate($items[$delta], $delta, $field, $entity, $instance, $langcode, $optional_field_found);
    }
  }

  if ($instance['settings']['url'] === 'optional' && $instance['settings']['title'] === 'optional' && $instance['required'] && !$optional_field_found) {
    form_set_error($field['field_name'] . '][' . $langcode . '][0][title', t('At least one title or URL must be entered.'));
  }
}


error: patch failed: link.module:263
error: link.module: patch does not apply