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.**
removed unnecessary assignment
updated phpDoc
Merge branch 'pr-1259-invalid-zip-codes-error' into branch-3.8.14.1-dev

Conflicts:
	wpsc-includes/shopping_cart_functions.php