pallet-scheduler: Send CallUnavailable event if the call isn't present (#1161)

The event was already send before, but it was done at the wrong
position. The pull request all changes the `execute_dispatch` signature
to highlight that there is only one error type.

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/frame/scheduler/src/tests.rs

@@ -1878,3 +1878,42 @@ fn reschedule_named_last_task_removes_agenda() {
 		assert!(Agenda::<Test>::get(when).len() == 0);
 	});
 }
+
+/// Ensures that an unvailable call sends an event.
+#[test]
+fn unavailable_call_is_detected() {
+	use frame_support::traits::schedule::v3::Named;
+
+	new_test_ext().execute_with(|| {
+		let call =
+			RuntimeCall::Logger(LoggerCall::log { i: 42, weight: Weight::from_parts(10, 0) });
+		let hash = <Test as frame_system::Config>::Hashing::hash_of(&call);
+		let len = call.using_encoded(|x| x.len()) as u32;
+		// Important to use here `Bounded::Lookup` to ensure that we request the hash.
+		let bound = Bounded::Lookup { hash, len };
+
+		let name = [1u8; 32];
+
+		// Schedule a call.
+		let _address = <Scheduler as Named<_, _, _>>::schedule_named(
+			name,
+			DispatchTime::At(4),
+			None,
+			127,
+			root(),
+			bound.clone(),
+		)
+		.unwrap();
+
+		// Ensure the preimage isn't available
+		assert!(!Preimage::have(&bound));
+
+		// Executes in block 4.
+		run_to_block(4);
+
+		assert_eq!(
+			System::events().last().unwrap().event,
+			crate::Event::CallUnavailable { task: (4, 0), id: Some(name) }.into()
+		);
+	});
+}