Add descriptive error when orchestration type is not registered

Replace NullPointerException with IllegalStateException containing a
descriptive message when a worker receives an orchestration event for a
type it doesn't have registered. This helps users understand that
workers that don't support the requested orchestration type are
connected to the task hub.

Author: torosent

client/src/main/java/com/microsoft/durabletask/TaskOrchestrationExecutor.java

@@ -882,7 +882,13 @@ private void processEvent(HistoryEvent e) {
                             // Try getting the default orchestrator
                             factory = TaskOrchestrationExecutor.this.orchestrationFactories.get("*");
                         }
-                        // TODO: Throw if the factory is null (orchestration by that name doesn't exist)
+                        if (factory == null) {
+                            throw new IllegalStateException(
+                                "No orchestration factory registered for orchestration type '" + name + "'. " +
+                                "This usually means that a worker that doesn't support this orchestration type " +
+                                "is connected to this task hub. Make sure the worker has a registered " +
+                                "orchestration for '" + name + "'.");
+                        }
                         TaskOrchestration orchestrator = factory.create();
                         orchestrator.run(this);
                         break;