Explicitly show the error message in dagfactory.py

[Skobnikoff]

Quite often I get the DAG compilation error and I have no clue what's wrong because the error message does not help at all:

Traceback (most recent call last):
  File "airflow/dags/main.py", line 14, in <module>
    dag_factory.generate_dags(globals())
  File "/usr/local/lib/python3.7/site-packages/dagfactory/dagfactory.py", line 126, in generate_dags
    dags: Dict[str, Any] = self.build_dags()
  File "/usr/local/lib/python3.7/site-packages/dagfactory/dagfactory.py", line 103, in build_dags
    ) from err
Exception: Failed to generate dag my_DAG. verify config is correct

I created a branch and tried to push it and submit a PR but got the access denied. No pb. Just please explicit the error message here. It could be as simple as this:
raise Exception("Failed to generate dag {dag_name}: {err}")

Many thanks!