Add error message if camera can't open

scripts/core/sxmo_camera.sh

@@ -1,3 +1,7 @@
 #!/usr/bin/env sh
+err() {
+  echo -e "$1" | dmenu -fn Terminus-20 -c -l 10
+}
+
 media-ctl -d /dev/media1 --set-v4l2 '"ov5640 3-004c":0[fmt:UYVY8_2X8/1280x720]'
-mpv --video-rotate=90 av://v4l2:/dev/video1
+mpv --video-rotate=90 av://v4l2:/dev/video1 || err "Couldn't open camera, is camera enabled?"