Provide a way to recover error types from UDFs

[sadderchris]

Currently, the UDF API returns a Box<dyn Error> to indicate failure. This is fine if you don't need to inspect errors once they come out of the PLR evaluator. However, it makes it really difficult to map errors that occurred inside a UDF to an external error type (since all type information is lost).

It would be nice to provide a way to recover the error type (e.g. via downcasting), like anyhow, or just adopting anyhow wholesale as the error type.