Treat non-deterministic prep errors as internal errors (#4364)

Closes https://github.com/paritytech/polkadot/issues/4293

This PR changes the way how we treat a certain subset of PVF preparation
errors. Specifically, now only the deterministic errors are treated as
invalid candidates. That is, the errors that are easily
attributable to either the the PVF contents or the wasmtime code, but
not e.g. I/O errors that could be triggered by the OS (insufficient
memory, disk failure, too much load, etc). The latter are treated as
internal errors and thus do not trigger the disputes.

polkadot/node/core/candidate-validation/src/lib.rs

@@ -459,6 +459,8 @@ async fn validate_candidate_exhaustive(
 			Ok(ValidationResult::Invalid(InvalidCandidate::ExecutionError(
 				"ambiguous worker death".to_string(),
 			))),
+		Err(ValidationError::InvalidCandidate(WasmInvalidCandidate::PrepareError(e))) =>
+			Ok(ValidationResult::Invalid(InvalidCandidate::ExecutionError(e))),
 
 		Ok(res) =>
 			if res.head_data.hash() != descriptor.para_head {