Strip the exception type when showing a checkArgument message to the user

[devinbileck]

When a checkArgument fails and raises an exception, it is shown to the user with the exception type (java.lang.IllegalArgumentException). It may be confusing to show that, and instead should just show the message.

Related to #2619

[devinbileck]

As suggested in #2614 (comment), a better implementation would be input validation where a more user friendly and translated error is shown (at text field validation).

[ManfredKarrer]

Excpetion class name is not displayed anymore as I use getMessage(). But still would be good to show a translated and more user friendly popup.

[ManfredKarrer]

Close as it is covered in #2619