media-export: Clarify message

Signed-off-by: Jens Georg <mail@jensge.org>
renderer-gst: Return null in error case

This is slightly bad since this is a constructor; we can safely return here
because the C code has not executed the constructor yet.

As the function was deprecated anyway with an earlier commit, beaty doesn't
really matter.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=722021