jack: tone down log ERRORs in case no JACK server is running

jackaudiosink and jackaudiosrc have a rank and might be plugged as part of auto-plugging inside playbin and playsink or the autoaudiosink/autoaudiosrc elements, so we don't really want to spew ERROR log messages in that case, which is consistent with what alsasink and pulseaudiosink do.

This is less noticable on Linux because pulseaudiosink has a higher and alsasink which has the same rank comes before jack in the alphabet.