Better error for missing TopRatedsMetadata (#2860)

libafl/src/schedulers/testcase_score.rs

@@ -312,7 +312,14 @@ where
             None => 0.0,
         };
 
-        let avg_top_size = state.metadata::<TopRatedsMetadata>()?.map().len() as f64;
+        let avg_top_size =
+            match state.metadata::<TopRatedsMetadata>() {
+                Ok(m) => m.map().len() as f64,
+                Err(_) => return Err(Error::key_not_found(
+                    "TopRatedsMetadata not found! You have to use Minimizer scheduler with this.",
+                )),
+            };
+
         weight *= 1.0 + (tc_ref / avg_top_size);
 
         if favored {