Fix AttributeError in ExtensionError

In Python 3, the attribute BaseException.message doesn't exist.

$ python3
>>> from sphinx.errors import ExtensionError
>>> e = ExtensionError('foo')
>>> repr(e)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sphinx/sphinx/errors.py", line 65, in __repr__
    return '%s(%r)' % (self.__class__.__name__, self.message)
AttributeError: 'ExtensionError' object has no attribute 'message'

sphinx/errors.py

@@ -55,6 +55,7 @@ class ExtensionError(SphinxError):
     def __init__(self, message, orig_exc=None):
         # type: (unicode, Exception) -> None
         SphinxError.__init__(self, message)
+        self.message = message
         self.orig_exc = orig_exc
 
     def __repr__(self):