rtmpproto: Print the error code string if there's no description

On (certain streams/setups at least on) Red5, the description string
actually is present, but empty. Therefore, first try loading the
description, but if not found or empty, load the code string instead.
The code string is quite understandable in most cases anyway (like
"NetStream.Play.StreamNotFound").

Signed-off-by: Martin Storsjö <martin@martin.st>