Fix several critical errors on redirect_form callback.

Billing address value() wasn't valid.
Added some checks to see if billing address exists before just trying
to populate non-existent array keys.
Limit buyerName to 100 characters after receiving an error from BitPay
after experimentation.
Fix debug watchdog for recording serialized options send to BitPay.
Add a watchdog to report an error and the message if invoice creation
fails.
Fix broken invoice URL redirection by giving the URL a proper variable
name to send to drupal_goto().