Refactor admin test form: error handling

The admin test form doesn't use a validation handler to check the entered asin. Therefore no form_set_error() is thrown on the asin field. Rather, a curious drupal_set_message of type "status" implies "Good news: The test failed."
I also refactored some code in order to improve the user experience a bit.
Finally, I separated out the lengthy error handling from _amazon_item_batch_lookup_from_web(), in order to keep the main functionality easy to grasp.