v4l2src: don't error in shutdown

Don't log with the ERROR category when we are stopping because we are shutting
down.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=672824

sys/v4l2/gstv4l2src.c

@@ -824,7 +824,8 @@ gst_v4l2src_fill (GstPushSrc * src, GstBuffer * buf)
   /* ERROR */
 error:
   {
-    GST_ERROR_OBJECT (src, "error processing buffer");
+    GST_DEBUG_OBJECT (src, "error processing buffer %d (%s)", ret,
+        gst_flow_get_name (ret));
     return ret;
   }
 }