Fixed PHP-690: Percentage symbol is not escaped in error messages

We need to use "%s", error_message when passing to the varargs function
mongo_cursor_throw. The explanation for why this is important is well described
at http://en.wikipedia.org/wiki/Uncontrolled_format_string
Merged pull request #303

* pull-request/303:
  Fixed PHP-690: Percentage symbol is not escaped in error messages