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