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`

首先跟踪报告中显示的 JWTAuth.from_settings_file 调用，并检查可选依赖 jwt 的加载方式。在不安装 boxsdk[jwt] 的情况下重现缺少依赖的情形，然后让错误提示建议运行 pip install boxsdk[jwt]，并验证安装该依赖后现有的 JWT 路径仍然有效。

由索引模型根据 Issue 内容生成。