Implements hook_field_validate().
Possible error codes:
function field_test_field_validate($entity_type, $entity, $field, $instance, $langcode, $items, &$errors) {
$args = func_get_args();
field_test_memorize(__FUNCTION__, $args);
foreach ($items as $delta => $item) {
if ($item['value'] == -1) {
$errors[$field['field_name']][$langcode][$delta][] = array(
'error' => 'field_test_invalid',
'message' => t('%name does not accept the value -1.', array(
'%name' => $instance['label'],
)),
);
}
}
}