Provide more specific error messages for date components

Thanks, this seems like a reasonable improvement. The code needs to be improved slightly for translation purposes. We shouldn't be trying to reassemble the word day/month/year from a non-translatable source. Instead we should give each field an internal label, or pull it from existing properties (like maybe #attributes['title']).

Thanks @spheresh! While testing your patch I found that it would skip the remainder of the validation (such as invalidating Feb 30) because a "return" was being called in all situations:

    // Check if the user filled the required fields.
    if($element['#required'] && count($errors) >= 3){
      form_error($element, t('!name field is required.', array('!name' => $element['#title'])));
      return;
    }
    else {
      foreach ($errors as $field_type) {
        $element_name = implode('][', array_merge($element['#parents'], (array) $field_type));
        // It will be marked only wrong items of fields, unlike to the form_error function that marks all items for the date field.
        form_set_error($element_name, t('!name field is not filled.', array('!name' => $element[$field_type]['#title'])));
      }
      return;
    }

I made the "else" an "elseif (count($errors))" to make it so the return is only called if there's a missing value.

I've committed this patch to the project. Please let me know if you encounter any issues.

Sorry this version was actually committed. The previous version has an issue with not throwing an error on a completely empty required date.

Needs porting to 8.x-4.x.

Committed and pushed 2a37061 and 12cf059 to 8.x-4.x. Thanks!