add transport action explicitly to test; improve assert error message

Author: DiannaHohensee

server/src/main/java/org/elasticsearch/transport/TransportService.java

@@ -1050,7 +1050,7 @@ private void sendLocalRequest(long requestId, final String action, final Transpo
             @SuppressWarnings("unchecked")
             final RequestHandlerRegistry<TransportRequest> reg = (RequestHandlerRegistry<TransportRequest>) getRequestHandler(action);
             if (reg == null) {
-                assert false : action;
+                assert false : "Action [" + action + "] not found";
                 throw new ActionNotFoundTransportException("Action [" + action + "] not found");
             }
             final Executor executor = reg.getExecutor();

server/src/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java

@@ -2510,6 +2510,10 @@ public RecyclerBytesStreamOutput newNetworkBytesStream() {
                 nodeConnectionsService = new NodeConnectionsService(clusterService.getSettings(), threadPool, transportService);
                 final ActionFilters actionFilters = new ActionFilters(emptySet());
                 Map<ActionType<?>, TransportAction<?, ?>> actions = new HashMap<>();
+                actions.put(
+                    TransportUpdateSnapshotStatusAction.TYPE,
+                    new TransportUpdateSnapshotStatusAction(transportService, clusterService, threadPool, snapshotsService, actionFilters)
+                );
                 actions.put(
                     GlobalCheckpointSyncAction.TYPE,
                     new GlobalCheckpointSyncAction(