[Errors] Make '_OperationalEvaluationError' work in the machines

[effectfully]

This replaces

instance AsEvaluationFailure err => AsEvaluationFailure (ErrorWithCause err cause) 

with

instance AsEvaluationError err structural operational =>
        AsEvaluationError (ErrorWithCause err cause) structural operational where

making error throwing more uniform in the machines (plus it adds functionality that I need in #7029).

Also makes throwingWithCause accept a term rather than a Maybe term, as the name indicates there really should be a cause.

Also replaces

throwing_ _EvaluationFailure

with

throwingWithCause _OperationalEvaluationError CekEvaluationFailure $ Error ()

in the machines, so that the logs clearly indicate that it was error that caused an evaluation failure -- not a builtin.

[zliu41]

It seems the key is to replace throwing_ with throwingWithCause - adding the cause. What's the purpose of using AsEvaluationError instead of AsEvaluationFailure? What's the relationship between these two?

[effectfully]

AsEvaluationError is more general, it allows one to throw both structural and operational errors. AsEvaluationFailure only allows one to throw the single operational error corresponding to an Error call. This is way too restrictive and I need other operational errors in the casing-on-booleans branch, so I figured I'd just forbid throwing Error this way, so that we uniformly throw all operational errors through AsEvaluationError.

Maybe we should replace _MachineError with _StructuralEvaluationError for consistency too, since the former is a synonym for the latter. Not sure. The former is kinda more readable and is shorter too.

[effectfully]

Maybe we should replace _MachineError with _StructuralEvaluationError for consistency too, since the former is a synonym for the latter. Not sure. The former is kinda more readable and is shorter too.

Or do the opposite and alias _OperationalEvaluationError as _UserError. Would also be more readable and shorter. Or maybe we should drop Evaluation from _StructuralEvaluationError and _OperationalEvaluationError. _StructuralError is perhaps even better than _MachineError.

[zliu41]

LGTM. The error handling could indeed use some more restructuring or refactoring. The naming - AsEvaluationError vs AsEvaluationFailure - doesn't help either.