fix nicer ImportError message on Python 3

Exception.message is not defined on Python 3

IPython/nbformat/validator.py

@@ -12,7 +12,7 b' except ImportError as e:'
     
     Please install it first.
     """
-    raise ImportError(e.message + verbose_msg)
+    raise ImportError(str(e) + verbose_msg)
 
 try:
     import jsonpointer as jsonpointer
@@ -23,7 +23,7 b' except ImportError as e:'
     
     Please install it first.
     """
-    raise ImportError(e.message + verbose_msg)
+    raise ImportError(str(e) + verbose_msg)
 
 from IPython.utils.py3compat import iteritems