Reduce shard started failure logging (#43330)

If the master is stepping or shutting down, the error-level logging can cause quite a bit of noise.

Author: ywelsch

server/src/main/java/org/elasticsearch/cluster/action/shard/ShardStateAction.java

@@ -589,7 +589,11 @@ public ClusterTasksResult<StartedShardEntry> execute(ClusterState currentState,
 
         @Override
         public void onFailure(String source, Exception e) {
-            logger.error(() -> new ParameterizedMessage("unexpected failure during [{}]", source), e);
+            if (e instanceof FailedToCommitClusterStateException || e instanceof NotMasterException) {
+                logger.debug(() -> new ParameterizedMessage("failure during [{}]", source), e);
+            } else {
+                logger.error(() -> new ParameterizedMessage("unexpected failure during [{}]", source), e);
+            }
         }
     }