fix nicer ImportError message on Python 3

Exception.message is not defined on Python 3

IPython/nbformat/validator.py

@@ -1,112 +1,112 b''
 from __future__ import print_function
 import json
 import os
 
 try:
     from jsonschema import SchemaError
     from jsonschema import Draft3Validator as Validator
 except ImportError as e:
     verbose_msg = """
 
     IPython depends on the jsonschema package: https://pypi.python.org/pypi/jsonschema
     
     Please install it first.
     """
-    raise ImportError(e.message + verbose_msg)
+    raise ImportError(str(e) + verbose_msg)
 
 try:
     import jsonpointer as jsonpointer
 except ImportError as e:
     verbose_msg = """
 
     IPython depends on the jsonpointer package: https://pypi.python.org/pypi/jsonpointer
     
     Please install it first.
     """
-    raise ImportError(e.message + verbose_msg)
+    raise ImportError(str(e) + verbose_msg)
 
 from IPython.utils.py3compat import iteritems
 
 
 from .current import nbformat, nbformat_schema
 schema_path = os.path.join(
     os.path.dirname(__file__), "v%d" % nbformat, nbformat_schema)
 
 
 def isvalid(nbjson):
     """Checks whether the given notebook JSON conforms to the current
     notebook format schema. Returns True if the JSON is valid, and
     False otherwise.
 
     To see the individual errors that were encountered, please use the
     `validate` function instead.
 
     """
 
     errors = validate(nbjson)
     return errors == []
 
 
 def validate(nbjson):
     """Checks whether the given notebook JSON conforms to the current
     notebook format schema, and returns the list of errors.
 
     """
 
     # load the schema file
     with open(schema_path, 'r') as fh:
         schema_json = json.load(fh)
 
     # resolve internal references
     schema = resolve_ref(schema_json)
     schema = jsonpointer.resolve_pointer(schema, '/notebook')
 
     # count how many errors there are
     v = Validator(schema)
     errors = list(v.iter_errors(nbjson))
     return errors
 
 
 def resolve_ref(json, schema=None):
     """Resolve internal references within the given JSON. This essentially
     means that dictionaries of this form:
 
     {"$ref": "/somepointer"}
 
     will be replaced with the resolved reference to `/somepointer`.
     This only supports local reference to the same JSON file.
 
     """
 
     if not schema:
         schema = json
 
     # if it's a list, resolve references for each item in the list
     if type(json) is list:
         resolved = []
         for item in json:
             resolved.append(resolve_ref(item, schema=schema))
 
     # if it's a dictionary, resolve references for each item in the
     # dictionary
     elif type(json) is dict:
         resolved = {}
         for key, ref in iteritems(json):
 
             # if the key is equal to $ref, then replace the entire
             # dictionary with the resolved value
             if key == '$ref':
                 if len(json) != 1:
                     raise SchemaError(
                         "objects containing a $ref should only have one item")
                 pointer = jsonpointer.resolve_pointer(schema, ref)
                 resolved = resolve_ref(pointer, schema=schema)
 
             else:
                 resolved[key] = resolve_ref(ref, schema=schema)
 
     # otherwise it's a normal object, so just return it
     else:
         resolved = json
 
     return resolved