Catch InvalidRequestError thrown by invoices.create

[meshy]

I've been getting "Nothing to invoice for customer" thrown by the create(customer) call, and this small change fixes the error.

The full response body I was getting from POST /v1/invoices is:

{
  "error": {
    "type": "invalid_request_error",
    "message": "Nothing to invoice for customer",
    "param": "customer"
  }
}

[coveralls]

Coverage remained the same at 100.0% when pulling 5ffd66a on wildfoundry:nothing-to-invoice into ec14b69 on pinax:master.