grandpa: minor cleanups in communication module (#6371)

* grandpa: replace Result<(), ()> with Option<()>

* grandpa: replace &Option<T> with Option<&T>

* grandpa: cleanup local id and keystore usages

* grandpa: return bool on check_message_signature

* grandpa: fix erroneous log message on startup

* grandpa: fix test

substrate/frame/grandpa/src/equivocation.rs

@@ -145,7 +145,7 @@ where
 
 		// validate equivocation proof (check votes are different and
 		// signatures are valid).
-		if let Err(_) = sp_finality_grandpa::check_equivocation_proof(equivocation_proof.clone()) {
+		if !sp_finality_grandpa::check_equivocation_proof(equivocation_proof.clone()) {
 			return Err(ReportEquivocationValidityError::InvalidEquivocationProof.into());
 		}