GUI Calculator errors

[ColdIV]

(GUI)
When I enter many numbers the border around the output will disappear.
The last 2 numbers will overlap the border.

(Error output) I do not receive any error messages. (For rubbish input like abcd or for math errors like 5/0) Maybe we should add an extra window for displaying errors?

See: #36

[laser-shark]

The border should not really be displayed anyway, it is just for testing purposes, I suppose we could wrap the display in a if(debug) statement. So I think the overlapping should not be a problem.