Raise ActionViewHelper errors.

@bslobodin /cc @Shopify/payments Simply raise ActionViewHelperErrors instead. Should we change the error messages to something else?
Would this ever be ``nil`` or throw a parse error instead for invalid JSON? It sounds like it would probably be indicative of some temporary issue with Coinbase and the message should recommend retrying a transaction? Note, these are shown to customers, so showing invalid JSON is probably not the best experience, so the message should be more user-friendly.