Rather than leak references to internal functions, we give an error message to the user explaining what dagster home is and how to set it.
Maybe after "...in your filesystem" we can add "that contains your dagster instance configuration file (dagster.yaml)"
I think we should also add an informative error if the user sets $DAGSTER_HOME correctly but there is no dagster.yaml present.
I think that by the time we enter this code path, we do require that the file exists so ""If nothing is specified..." section would be a bit misleading.
Was thinking we should do something similar to check.invariant(os.path.isdir(base_dir), "base_dir should be a directory") which raises on failure