[7.0] Add a more helpful error message when validator init fails

[PaintNinja]

Fixes #109. Thanks to @basdxz for finding the root cause of the issue.

[LexManos]

Instead of throwing an error, would it be better to allow the annotation processor to be applied (since we do it by default in the mdk) and just disable itself if it detects this case?

[PaintNinja]

Possibly, but you'd still be paying the overhead of the compiler setting up the AP environment and sending each source file to it.

The MDK is unaffected by this issue as far as I'm aware, as Forge transitively includes EventBus and there's no module-info in the MDK. To trigger the issue, you need to run the AP on a project that is missing EventBus or where the project being validated has an explicit module but missing requires statement.

[LexManos]

Fair enough. Haven't looked into this part of the compiler so not sure how much of a cost it is vs having no APs.