Failure message in assertGenerationConsistency (#108908)

If this assertion trips it would be useful to report these details.

Relates #108907

Author: DaveCTurner

server/src/main/java/org/elasticsearch/snapshots/InFlightShardSnapshotStates.java

@@ -97,7 +97,8 @@ private static boolean assertGenerationConsistency(
         @Nullable ShardGeneration activeGeneration
     ) {
         final ShardGeneration bestGeneration = generations.getOrDefault(indexName, Collections.emptyMap()).get(shardId);
-        assert bestGeneration == null || activeGeneration == null || activeGeneration.equals(bestGeneration);
+        assert bestGeneration == null || activeGeneration == null || activeGeneration.equals(bestGeneration)
+            : "[" + indexName + "][" + shardId + "]: " + bestGeneration + " vs " + activeGeneration;
         return true;
     }