`CurvesAsSubmobjects` error messages contain hardcoded class names

The class CurvesAsSubmobjects throws two custom error messages for when it either has no submobjects or it has no submobjects with points:

https://github.com/ManimCommunity/manim/blob/cafc63919eb03d398d7038a8c82e591b12de8da5/manim/mobject/types/vectorized_mobject.py#L2858-L2874

I think it would be a good idea for these error messages to dynamically show the name of the class rather than a hardcoded "CurvesAsSubmobjects" string so the error message automatically updates for any subclasses or if the class name ever changes.