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/primitives/npos-elections/src/lib.rs

@@ -119,12 +119,14 @@ pub enum Error {
 	SolutionTargetOverflow,
 	/// One of the index functions returned none.
 	SolutionInvalidIndex,
-	/// One of the page indices was invalid
+	/// One of the page indices was invalid.
 	SolutionInvalidPageIndex,
 	/// An error occurred in some arithmetic operation.
 	ArithmeticError(&'static str),
 	/// The data provided to create support map was invalid.
 	InvalidSupportEdge,
+	/// The number of voters is bigger than the `MaxVoters` bound.
+	TooManyVoters,
 }
 
 /// A type which is used in the API of this crate as a numeric weight of a vote, most often the