[KFileWidget] Use KMessageWidget for error reporting

Instead of a KMessageBox. It's a lot nicer :)

(Didn't dare touching KDirOperator, though)

Seems to reliably go away when navigating to a different folder (not entirely sure under which circumstances urlEntered is and isn’t emitted)

I wonder if KDirOperator should emit canceled when jobError occurs?

The error message boxes ("can only select one file", "file could not be found", etc) when accepting the dialog are kept.

@dfaure @ahmadsamir