drupal_set_message should be used on in UI related code

Using drupal_set_message should be reserved to UI related code, like forms.

If an API function that can/would be called programmatically, it should not contain drupal_set_message because their is no guarantee that it is being called from the UI. So instead of putting the drupal_set_message call in the API function put it in the form function that is guaranteed to be called by the UI.

Examples can be found in the format plugins.