Fix asin.module's error handling

Some things are wrong with the asin field:
When an invalid ASIN is given, the following errors are thrown:

"Warning: htmlspecialchars() expects parameter 1 to be string, array given in check_plain() (line 1476 of /var/www/includes/bootstrap.inc).
No Amazon product with the ASIN "" could be located."

That's not exactly what we want.
Reason is that _asin_load_items() takes $items by reference and cryptically manipulates on it, even though it returns everything needed. As this helper function is being called only once and is overly abstracted, I removed it altogether.

Also, the form_set_error on the ASIN field does not work.
Finally, I refactored asin_field_formatter_view