This changes the following keyframing functions to return
`SingleKeyingResult`, which is in turn used for better failure
reporting in the higher-level functions that call them:
- `KeyframeStrip::keyframe_insert()`
- `insert_vert_fcurve()`
As a side effect, this also means that
`rna_KeyframeAnimationStrip_key_insert()` can no longer return an
`FCurve *`, and now instead returns a bool indicating success.
This is part of an ongoing progressive refactor to improve error
messages and failure handling in the keyingframing code.
Pull Request: https://projects.blender.org/blender/blender/pulls/121517
8dbd167e80