[mlir] Better error message in PybindAdaptors.h

When attempting to cast a pybind11 handle to an MLIR C API object through
capsules, the binding code would attempt to directly access the "_CAPIPtr"
attribute on the object, leading to a rather obscure AttributeError when the
attribute was missing, e.g., on non-MLIR types. Check for its presence and
throw a TypeError instead.

Depends On D117646

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D117658