VimError: set self.message

`self.message` is used in `__str__`.

Without this, tracebacks end like this:

    Traceback (most recent call last):
    [...]
      File "…/vim/neobundles/jedi/jedi_vim.py", line 87, in _catch_
    exception
        raise VimError(result['exception'], result['throwpoint'], string)
    jedi_vim.VimError: <unprintable VimError object>

jedi_vim.py

@@ -68,6 +68,7 @@ def _check_jedi_availability(show_error=False):
 class VimError(Exception):
     def __init__(self, message, throwpoint, executing):
         super(type(self), self).__init__(message)
+        self.message = message
         self.throwpoint = throwpoint
         self.executing = executing