Updated JSON error message to limit full error to 80 characters.

Also added a comment to explain why.

IPython/nbformat/reader.py

@@ -40,7 +40,8 b' def parse_json(s, **kwargs):'
     try:
         nb_dict = json.loads(s, **kwargs)
     except ValueError:
-        raise NotJSONError("Notebook does not appear to be JSON: %r" % s[:16])
+        # Limit the error message to 80 characters.  Display whatever JSON will fit.
+        raise NotJSONError(("Notebook does not appear to be JSON: %r" % s)[:77] + "...")
     return nb_dict
 
 # High level API