Fix and refactor amazon_store_report_error()

1.) PHP complains about the undefined variables $i and $msg in amazon_store_report_error().
2.) <br/> is being printed in plain instead of starting a new line.
3.) According to watchdog()'s docs, $type should be the name of the calling module, i.e. "amazon_store" not "amazon".
4.) Finally, the error message text "Error processing SimpleXML" is not very useful, because the Amazon query has failed, while the XML was indeed successfully processed.
Allowing for a custom message text would make the function more flexible, so it can be used at least at two more places.