Offence reporting returns a result (#5082)

* Offence reporting returns a result

* Bump spec_version

* Use unwrap instead of assertions

* Fix more review grumbles

substrate/primitives/staking/src/offence.rs

@@ -91,14 +91,37 @@ pub trait Offence<Offender> {
 	) -> Perbill;
 }
 
+/// Errors that may happen on offence reports.
+#[derive(PartialEq, sp_runtime::RuntimeDebug)]
+pub enum OffenceError {
+	/// The report has already been sumbmitted.
+	DuplicateReport,
+
+	/// Other error has happened.
+	Other(u8),
+}
+
+impl sp_runtime::traits::Printable for OffenceError {
+	fn print(&self) {
+		"OffenceError".print();
+		match self {
+			Self::DuplicateReport => "DuplicateReport".print(),
+			Self::Other(e) => {
+				"Other".print();
+				e.print();
+			}
+		}
+	}
+}
+
 /// A trait for decoupling offence reporters from the actual handling of offence reports.
 pub trait ReportOffence<Reporter, Offender, O: Offence<Offender>> {
 	/// Report an `offence` and reward given `reporters`.
-	fn report_offence(reporters: Vec<Reporter>, offence: O);
+	fn report_offence(reporters: Vec<Reporter>, offence: O) -> Result<(), OffenceError>;
 }
 
 impl<Reporter, Offender, O: Offence<Offender>> ReportOffence<Reporter, Offender, O> for () {
-	fn report_offence(_reporters: Vec<Reporter>, _offence: O) {}
+	fn report_offence(_reporters: Vec<Reporter>, _offence: O) -> Result<(), OffenceError> { Ok(()) }
 }
 
 /// A trait to take action on an offence.