Enforce MaxEncodedLen impl for NposSolution (#11103)

* Fail if `MaxVoters` too small

* Fixing benchmarking test, better naming of error

* reverting accidental change

* use fully qualified syntax
no need to interate to calculate len

* Fail directly if too many voters

substrate/frame/election-provider-multi-phase/src/mock.rs

@@ -74,7 +74,7 @@ frame_election_provider_support::generate_solution_type!(
 		VoterIndex = VoterIndex,
 		TargetIndex = TargetIndex,
 		Accuracy = PerU16,
-		MaxVoters = ConstU32::<20>
+		MaxVoters = ConstU32::<2_000>
 	>(16)
 );