Fix error message when failing to load a notebook

- fix missing import in nbformat.reader.reads
- show JSON error message if there is one

Before, opening a v4 notebook would show:

> Error: Bad Request

After, it shows the intended:

> Unreadable Notebook: /path/to/notebook.ipynb Unsupported nbformat version 4

Should be backported to 2.x
I see this same logic here and above in utils.js. Do we want a function in utils.js that takes an xhr and returns a properly selected error message?