sys/v4l2/: Cleanup error messages and unify header comments

sys/v4l2/: Cleanup error messages and unify header comments

Original commit message from CVS:
* sys/v4l2/gstv4l2.c:
* sys/v4l2/gstv4l2colorbalance.c:
* sys/v4l2/gstv4l2colorbalance.h:
* sys/v4l2/gstv4l2object.c:
* sys/v4l2/gstv4l2object.h:
* sys/v4l2/gstv4l2src.c:
* sys/v4l2/gstv4l2src.h:
* sys/v4l2/gstv4l2tuner.c:
* sys/v4l2/gstv4l2tuner.h:
* sys/v4l2/gstv4l2xoverlay.c: (gst_v4l2_xoverlay_open):
* sys/v4l2/gstv4l2xoverlay.h:
* sys/v4l2/v4l2_calls.c: (gst_v4l2_get_capabilities),
(gst_v4l2_open):
* sys/v4l2/v4l2_calls.h:
* sys/v4l2/v4l2src_calls.c: (gst_v4l2src_queue_frame),
(gst_v4l2src_capture_init):
* sys/v4l2/v4l2src_calls.h:
Cleanup error messages and unify header comments