Raise an error if no unprocessed payments.

Fixes #5436

/cc @athal7 This breaks an exchanges rake task spec spec.
I spent a bunch of time trying to make it work right,
but haven't figure out proper way to do it.  It appears the while order.next
loop completes order because of default credit card bug.  Basically need a solution
to: Refs #5327

Conflicts:
	core/app/models/spree/order/payments.rb
	core/spec/models/spree/order/payment_spec.rb