use new message() function in BitcoinGUI

- use it for displaying URI parsing warnings
- use it for displaying error and information in backup wallet function
  (the information display is new and the error was a warning before)

- cleanup BitcoinGUI::incomingTransaction()
-- use message() + the information icon from message
-- comment out an unused parameter in the function definition and
   declaration
-- move all pre-checks at the beginning of the function

src/qt/bitcoingui.h

@@ -168,7 +168,7 @@ private slots:
 
         The new items are those between start and end inclusive, under the given parent item.
     */
-    void incomingTransaction(const QModelIndex & parent, int start, int end);
+    void incomingTransaction(const QModelIndex& parent, int start, int /*end*/);
     /** Encrypt the wallet */
     void encryptWallet(bool status);
     /** Backup the wallet */