add missing NBFormatError import in nbformat.reader.reads

opening unreadable notebook format failed with 'NameError: NBFormatError'
instead of the expected error.

IPython/nbformat/reader.py

@@ -80,6 +80,8 b' def reads(s, **kwargs):'
     nb : NotebookNode
         The notebook that was read.
     """
+    from .current import NBFormatError
+    
     nb_dict = parse_json(s, **kwargs)
     (major, minor) = get_version(nb_dict)
     if major in versions: