[Qt] ensure cert errors lead to rejecting payment requests

- extend PaymentRequestPlus::getMerchant to return status of payment
  request errors related to certificates/PKIs etc.
- do a real check for errors, which was missing until now, in
  PaymentServer::processPaymentRequest and also extend our tests in
  paymentservertests.cpp to not only rely on the content of "merchant"
  passed to getMerchant
- this explicitly treats insecure payment requests not as errors, as they
  are allowed as per BIP70