[Qt] add messages when handling local payment request files

- important for the open URI dialog to give users feedback
  when a file is invalid etc.

src/qt/paymentserver.h

@@ -98,7 +98,7 @@ public slots:
     // Submit Payment message to a merchant, get back PaymentACK:
     void fetchPaymentACK(CWallet* wallet, SendCoinsRecipient recipient, QByteArray transaction);
 
-    // Handle an incoming URI or file
+    // Handle an incoming URI, URI with local file scheme or file
     void handleURIOrFile(const QString& s);
 
 private slots: