remoting: Handle non-fatal errors in gst_parse_launch

gst_parse_launch can return non-NULL even though an error is set. This
indicates "a recoverable parsing error and you can try to play the
pipeline", however given that we don't (and likely can't) make any
attempt to correct the situation, we should treat this as fatal and not
try to carry on.

Signed-off-by: Ray Smith <rsmith@brightsign.biz>