Fix errors and warnings when loading Skeleton2D Modifications

[johnsonbaugh]

Fixes #73247

• Don't print cache errors about not being "properly" setup when not setup, because that is normal
• Don't print warnings about verification when not setup
• Avoid cache warnings and other tree-reference-related errors calling setup_modifications on NOTIFICATION_POST_ENTER_TREE

[0xcafeb33f]

I tested this patch locally on my own project, and it resolved all of the incorrect error messages for my Skeleton2Ds.

Can this be cherrypicked to 4.2?

[lyuma]

I don't have much experience with using SkeletonModifier2D in production, but I am aware of the logspam issue it has, so this would be good to get in.

The change looks good. Almost all changes are making log messages less noisy which is a good thing.

Other than a few extra calls to fabrik_joint_update_bone2d_cache() and inside_tree checks for setup (and a NOTIFICATION_POST_ENTER_TREE to trigger that setup at the correct time) there are no functional changes so this seems pretty safe.

[akien-mga]

@johnsonbaugh Would you be able to rebase to solve the merge conflicts?

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉