Warn about supported datatypes

A potential new contributor was going through the getting started docs, and accidentally used `dot_product_8f.mlir` instead of `dot_product_8.mlir` with the `mlir-openfhe-to-bgv` pipeline. The lowering failed for obvious reasons (bgv doesn't lower float ops) but the failure was a generic message about failing to lower secret.generic when it was marked illegal.

We should have a pass that validates the pipeline supports the input before trying to lower anything.