[Qt] add verifySize() function to PaymentServer

- add static verifySize() function to PaymentServer and move the logging
  on error into the function
- also use the new function in the unit test
- the function checks if the size is allowed as per BIP70

src/qt/test/paymentservertests.cpp

@@ -185,7 +185,8 @@ void PaymentServerTests::paymentServerTests()
     tempFile.open();
     tempFile.write((const char*)randData, sizeof(randData));
     tempFile.close();
-    QCOMPARE(PaymentServer::readPaymentRequestFromFile(tempFile.fileName(), r.paymentRequest), false);
+    // compares 50001 <= BIP70_MAX_PAYMENTREQUEST_SIZE == false
+    QCOMPARE(PaymentServer::verifySize(tempFile.size()), false);
 
     // Payment request with amount overflow (amount is set to 21000001 BTC):
     data = DecodeBase64(paymentrequest5_cert2_BASE64);