makePayment() fails silently when given bad Body params

[raychz]

Like #2, we need to do better error handling, and return to the client a response that makes sense. When sending a POST to /payment with bad vals, I get a 201 with an empty response. Logging the response from Spreedly is a different story though:

{ errors: [ { key: 'errors.payment_method_or_payment_method_token_required',
       message: 'You must specify a valid \'payment_method\' or valid \'payment_method_token\' parameter.' } ] 
}