Output warning when falling back to default data manager

If an error occurs during loading a custom GK data manager, GK falls back to the default data manager. [This is done silently.](https://github.com/deepgenomics/GenomeKit/blob/4526888054c0b258b6f93a0877e1e8ba92010aaa/genome_kit/gk_data.py#L41-L45)
Suggestion: Output a warning log that this has occured.