paymentserver: style fixes / cleanup

- rename reportError() into message() to be in line with our default
  message() signal/slot naming (and can be used for all types of messages)
- rename some QStrings to not collide with message() function
- add a missing message for malformed URIs that IS also used in BitcoinGUI
- fix / extend some comments and misc style fixes

src/qt/paymentserver.h

@@ -90,8 +90,8 @@ signals:
     // Fired when a valid PaymentACK is received
     void receivedPaymentACK(QString);
 
-    // Fired when an error should be reported to the user
-    void reportError(QString, QString, unsigned int);
+    // Fired when a message should be reported to the user
+    void message(const QString &title, const QString &message, unsigned int style);
 
 public slots:
     // Signal this when the main window's UI is ready