Replace MessageBox with a custom UserNotifier

At this moment when a message must be send to the user (including errors/exceptions) a MessageBox is showed. The issues involved are:

• the code is harder to modify/maintain (let's say in the future we would like a custom Form to display user messages)
• it's difficult to debug: a central point where all the messages are showed + stack trace is better than a lot of messageboxes that must be analysed
  An UserNotifier class with some Error(), Warning() , Question() and Message() methods can be a solution.