Provide feedback when accessing undeclared outlet at runtime

### Description Currently, operations toward a non-declared outlet to `outlet_events` is simply ignored: ```python @task def emit_alias_event(): context = get_current_context() context["outlet_events"][AssetAlias("a")].add(Asset("new")) # Line above does nothing since AssetAlias("a") is not an outlet. ``` The same also applies to `yield Metadata`. This is a bit confusing to the user since they did something, and reasonably expect it’s an Airflow bug if nothing seems to happen. I can think of a couple of ways to resolve this: #### Automatically add outlets when you do this If you access something, it automatically becomes an outlet. However, Airflow would not be able to detect such a dynamic outlet in many other places, such as the dag dependency graph (which is generated statically). Would this only shift the confusion elsewhere? #### Raise a KeyError Or a more detailed error with a hint. More explicit, but would it be awkward for certain workflows? ### Use case/motivation _No response_ ### Related issues _No response_ ### Are you willing to submit a PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)