Don't display invalid shipping choices when there is a USPS quote error

* capture error from XML
* save error for next time checkout page is rendered
* output error (using existing action hooks ) into checkout page
* clear saved errors prior to shipping being recalculated

**Note that this is a quick (but reasonable) fix and needs to be expanded on going forward.**

SHould resolve issue #1259 until we get the more robust error handling of the new theme engine