pvf-precheck: teach runtime-api to work with versions (#4510)

As part of #3211 we will need to add a couple of runtime APIs. That
change is coming in a following PR.

The runtime API for pre-checking will be introduced with a version bump
for `ParachainHost`. This commit prepares the ground for that change,
by introducing a error condition that signals that the given API is not
supported.

polkadot/node/core/av-store/src/tests.rs

@@ -240,7 +240,18 @@ fn runtime_api_error_does_not_stop_the_subsystem() {
 				RuntimeApiRequest::CandidateEvents(tx),
 			)) => {
 				assert_eq!(relay_parent, new_leaf);
-				tx.send(Err(RuntimeApiError::from("oh no".to_string()))).unwrap();
+				#[derive(Debug)]
+				struct FauxError;
+				impl std::error::Error for FauxError {}
+				impl std::fmt::Display for FauxError {
+					fn fmt(&self, _f: &mut std::fmt::Formatter) -> std::fmt::Result {
+						Ok(())
+					}
+				}
+				tx.send(Err(RuntimeApiError::Execution {
+					runtime_api_name: "faux",
+					source: Arc::new(FauxError),
+				})).unwrap();
 			}
 		);