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
Added `utils.ajax_error_msg(jqXHR)` and used it in both places.