Add a better error handling for loading keyboard layouts

Currently there is error handling for keyboard layout related problems.

In the attached example the keyboard is shown although no Item has the focus.
If you change to the zh_TW layout in the keyboard, the layout is not loaded and you only see a blank keyboard without a way to change back to a working layout.

Something similar can happen if the actual layout contains some broken QML code or uses a corrupted QML cache. Both are errors which should be fixed independently, but it would be great if the keyboard is still usable with all other layouts.