adds content to 500 error. Rework of _container_config.

This PR adds the following:

  * 500 errors now include the content (if content exists) in the error message to make debugging easier.
  * the method _container_config has been reworked so that only non-empty or values set to True will be passed in the post to the server. This avoids most of the conversion issues (though there are a few fringe cases that it doesn't yet address) causing the 500 error responses.