Allow user DAGs to emit DagWarning (or similar)

### Description I would like a way to emit a warning or error message from a DAG that shows up as a banner in the web UI, similar to how exceptions are handled during DAG parsing or how [`DagWarning`](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/models/dagwarning/index.html)s are displayed. ### Use case/motivation I have a script under my `dags_folder` that is dynamically discovering and building DAGs from a networked filesystem. If one dynamically discovered DAG raises an exception, I don't want that to affect any of the other DAGs that are being discovered, so I swallow exceptions and log about them. However, I have not found a good way to communicate to my users that that exception occurred. Ideally I think it should look similar to when an exception is raised in a traditional DAG - an error banner on the web with (optionally) the full traceback. In my testing, I've found that if *any* exception is raised during DAG parsing, it causes *all* DAGs originating from that file to be removed. So this sort of thing does not work: ``` #!/usr/bin/env python3 from datetime import datetime import traceback from airflow.decorators import dag, task exceptions = [] for dag_id in 'ABCDE': try: @dag( dag_id=dag_id, schedule=None, start_date=datetime(2023, 1, 31), ) def test_multi_exception(): if dag_id in 'BE': raise RuntimeError(f'Failed to create dynamic DAG {dag_id}') @task def t(): print('inside task') t() test_multi_exception() except Exception as e: exceptions.append(''.join(traceback.format_exception(etype=type(e), value=e, tb=e.__traceback__))) if exceptions: raise RuntimeError('\n'.join(exceptions)) ``` ### Related issues _No response_ ### Are you willing to submit a PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Con…