explicitly validate compute_at and store_at during lowering

This gives more helpful error messages than missing symbols, or store_at
level not found, etc. It also lists the legal options, which should help
if you think you realized a function but didn't.
explicitly validate compute_at and store_at during lowering

This gives more helpful error messages than missing symbols, or store_at
level not found, etc. It also lists the legal options, which should help
if you think you realized a function but didn't.