plasmaappletitemmodel: Fix confusing wording in error message

Reason for the change

The error message stated that the widget is not compatible with the latest version of Plasma, and the user was prompted to update. This is confusing – The actual error is a mismatch in the minor version number of Plasma.

I think the last two error cases could be merged together, since the way to fix them is the same. Or am I missing something?