image_create_node_from should return a readable error

This bug report relates to a discussion that bwynants and I are having in the image_pub issue queue at #270085: Port to Drupal 6.

The question is whether to create an image node using image_create_node_from() or something like this code, which uses the form api drupal_execute() function:

$form_state = array(); 
module_load_include('inc', 'node', 'node.pages');
$node = array('type' => 'image');
$form_state['values']['title'] = $caption;
$form_state['values']['uid'] = $user->uid;
$form_state['values']['name'] = $user->name;
$form_state['values']['body'] = $description;
$form_state['values']['op'] = t('Save');
$redirect_url = drupal_execute('image_node_form', $form_state, (object)$node);
 

The advantage of image_create_from() over using drupal_execute() is that we think the name of the image_create_node_form() function is less likely to change than the names of the fields in the image_form.

The disadvantages of image_create_from() versus using drupal_execute() are that:
1. image_create_from() does not return any information about the error (as mentioned in comment #1), and that
2. it does not appear to check the sizes of the images.

On a related note - pardon my ignorance here, but does image_create_node_from really need to check for user permissions at all? I ask because I'm trying to call it from a cron hook in a custom module I'm developing, and this is causing me a bit of a headache because it's not clear what user, if any, cron runs as; if I manually run cron then it has root permissions and all is well, but when cron runs automatically it generates access_denied errors. image_create_node_from is not a function that ever gets directly called by a user, and it seems to me that it would make more sense to be checking for proper user permissions outside this function, in whatever functions call it (image_attach_validate, for instance); that would make this function itself more flexible in how it can be used by other modules (for instance, in cron hooks...).

you could make the argument either way about checking the permissions but i think bwynants is correct that it should return FALSE.

committing the attached to HEAD.