Warn about supported datatypes

[j2kun]

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.

[AlexanderViand-Intel]

I noticed this similar pattern a few times in other settings, too*. I think rather than doing this with a dedicated pass (which would only work for certain pipelines and only for the input parts of that pipeline), we should add a "fail on unknown stuff" pattern to all the relevant passes.

For example, in secret-to-whatever, this pattern should match (with lower priority, if we can do that in a dialect conversion?) any secret.generic op and produce a nice error message about how the op inside wasn't recognized by the pass and is therefore likely not in the set of supported inputs.

Kind of related to #1143

.* Specifically, I noticed that if you lower something unsupported to openfhe, it'll also just give you an error message about failing to lower illegal ops, rather than admitting that it's unsupported. However, this issue is pretty much universal.

(another) EDIT: I'm also wondering if we can return the InflightDiagnostic (emitError) directly instead of a "failure" in order to surpress the default DialectConversion error about failing to lower an op marked as illegal.

[j2kun]

I generally tend to prefer input validation over JIT-style error checking, but you're right that the diagnostic system is supposed to make this more pleasant. So I'll try the low-priority pattern idea.