Improved error message for unavailable variant

[theely]

If a kernel is not available, the raised exception now also includes the name of the required variant.
This change should help users to identify what variant they need for their system.

[danieldk]

Thanks for contributing to kernels!

Enumeration of variants is always going to be incomplete, see: #422 E.g., in the PR

        missing_variant.filename = f"torch{torch_version.major}{torch_version.minor}-{'cxx11' if torch_cxx11_abi else 'cxx98'}-cu{selected_backend.version.major}{selected_backend.version.minor}-{cpu}-{os}"

Misses: tvm-ffi, the variant without the ABI tag (which is also supported), backwards CUDA compatibility (e.g. if the system has CUDA 12.8, CUDA 12.1 to 12.7 are also supported), etc. In this case it would also report the a variant for cu if the backend is ROCm, XPU, NPU, CPU, Metal, etc.

We discussed this a while back and came to the conclusion that we are instead going to add support for 'explaining', where for every available variant it's possible to get/list the reason why it is not selected.