Not setting bucket name causes confusing error

[odigity]

I'm learning django-storages by starting with zero configuration and adding what's needed one bit at a time to see how it alters behavior.

When I configure credentials but not a default bucket, then make a request to the server, it fails with a ValueError.

This stops happening as soon as I either set settings.AWS_STORAGE_BUCKET_NAME or set bucket_name in my storage subclass.

It may be worth adding an explicit check for required settings in order to provide a relevant error message informing the user of where they're falling short.

Traceback: https://gist.github.com/odigity/d73cf6338a5bb7908399cbe58d84572d

[sevdog]

This should be reopened since #1322 reverted the fix for this.