Missing station data error is not handled well

[rod-glover]

Symptoms: Local deployment, which in my case lacks station data, throws a not very helpful error which is not caught and causes the app to misbehave.

Error message from dev mode:

Traceback (most recent call last):
  File "/codebase/dve/callbacks/map_figure.py", line 332, in update_map
    df = get_data_object(
AttributeError: 'NoneType' object has no attribute 'data_frame'

The data object being gotten is the station data.

Note: Production deployment has these files, but in case it doesn't it would be good to display a helpful error message instead of just screw up.

[rod-glover]

Note: This happened because I lost my VPN connection to /storage, and so couldn't access a number of files. That will be worth testing more thoroughly.