Prevent Display Indexing Out of Range Error for KoToolBox::buttonSize

Something about my specific development environment was causing a crash due
to QGuiApplication::screens() indexing throwing an Out of Range error.
It likely has something to do with my vertical-right oriented display I
just set up. I'll do more investigation of this issue, and whether its a
QT or GNOME bug. For now, I think preventing a hard crash for simple GUI
button size scaling is probably appropriate. If buttonSize is called
without a valid index, it will simply default to the highest button size
option. In order to track future occurances, it will now print warnings
when an invalid screen index is used.