add missing NBFormatError import in nbformat.reader.reads

opening unreadable notebook format failed with 'NameError: NBFormatError'
instead of the expected error.
display JSON error message on failed notebook load

if there is one (escaped for safety).
add utils.ajax_error_msg for extracting the JSON error message.
Merge pull request #6303 from minrk/nbformat-error

Fix error message when failing to load a notebook