Detect errors when creating a Source in the 3DS example

[danj-stripe]

Summary

Reporting errors creating the Source, and fixing crash that happens when source.cardDetails == nil.

Motivation

Two problems:

• This error was never being checked and reported
• When source or source.cardDetails is nil, source.cardDetails.threeDSecure == STPSourceCard3DSecureStatusRequired
  because STPSourceCard3DSecureStatusRequired == 0 and Obj-C's nil -> 0 conversion.

This resulted in passing a nil card id into the threeDSecureParamsWithAmount:... method,
which crashed when it tried to put it into an NSDictionary

Testing

This is the example code, so it doesn't have automated testing. Manually tested by using
a livemode publishable key (pk_live_...) and the auto-filled test card #, which
causes the initial Source creation to fail:

{
  "error": {
    "code": "card_declined",
    "doc_url": "https://stripe.com/docs/error-codes/card-declined",
    "message": "Your card was declined. Your request was in live mode, but used a known test card.",
    "type": "invalid_request_error"
  }
}