Clean up stacktrace for unimplemented class / method / field errors.

[cittyinthecloud]

This keeps stack traces resulting from unimplemented classes and members from being full of confusing Mixin and class loading errors.

[rikka0w0]

I would like to suggest that we should include a link for the users to submit their issue, for example, the github issue page.

[florensie]

Maybe check if the class/method/field is actually in the forge package?

[kitlith]

Forge adds methods and fields to vanilla classes, too, @florensie

[florensie]

@kitlith Fair but what if it's something else like a class missing in a dependency mod. I guess it might be too much effort to check every package forge touches.

[kitlith]

@florensie we could limit to net.minecraftforge and net.minecraft then? That's still not perfect if a mod adds a class under net.minecraft to access package private things, but it's something.

[TheGlitch76]

We don't let mods add things under the net.minecraft package, yet