Display error message

[arnaudroques]

Right now, error message are hidden when the user gives incorrect data.
This is fine somehow because it does not break the last working diagram display.

On the other hand, displaying the error message may help the user to understand what is going on.

What about displaying the generated error message in some area which is normally hidden (for example, on the bottom of the screen).
This area will be hidden back automatically when user's input will be correct.