Report helpful error message for missing "jwt" dependency

### Is your feature request related to a problem? Please describe.

As is known from closed issue (https://github.com/box/box-python-sdk/issues/387) if you don't `pip install "boxsdk[jwt]"`, and use the JWT authorization, you get a somewhat confusing message:

```
File "/Users/first.last/workspace/repo_name/box/../lib/box.py", line 70, in get_jwt_client
auth = boxsdk.JWTAuth.from_settings_file(load_config(self.app_name))
AttributeError: 'NoneType' object has no attribute 'from_settings_file'
```

### Describe the solution you'd like

Return an error message instead that at least suggests "Did you try pip install boxsdk[jwt]?" when JWTAuth isn't going to work.

### Additional context
Using sdk version `boxsdk 2.9.0`

Start by tracing the JWTAuth.from_settings_file call shown in the report and inspect how the optional jwt dependency is loaded. Reproduce the missing-dependency case without installing boxsdk[jwt], then make the failure suggest pip install boxsdk[jwt] and verify that the existing JWT path still works when the dependency is installed.

Written by the indexing model from the issue text.