Include specific error details in failed requests

[yazinsai]

A failed response currently returns a generic error message. e.g.

Request:

curl https://api.verifypayments.com/bank_accounts \
  -H "Authorization: Bearer test_sk_1bcdc09898675d6629faa19db108825d" \
  -d "sender={SENDER_ID}" \
  -d "name=Abdullah Mohamed Alahmed" \
  -d "iban=AE070331234567890123456" \
  -d "currency=AED"

Response:

{"error":{"type":"processing","code":"bad_request","message":"Bad Request","extras":{"time":"2018-03-14T08:18:23Z","request_id":"d60a4858-1757-4a78-ab18-5d81be7d39b5"}}}

The response should mention the specific error encountered (for the example above, that would be "Invalid sender" or something to that effect).