Handle missing/invalid Flickr API key

[mvaaltola]

Is your feature request related to a problem? Please describe.
Leaving the placeholder Flickr API key/secret in .env results in:

--- Importing Flickr data for Helsinki ---
Traceback (most recent call last):
  File "./import.py", line 85, in <module>
    bbox=",".join(bbox)
TypeError: sequence item 0: expected str instance, float found

This happens after running docker-compose run notebook ./import.py Helsinki.

Describe the solution you'd like
Handle the invalid API key gracefully, i.e. provide some error message.