vdl : add checks for ffmpeg and ffprobe.

.linux/bash/module/vdl.sh

@@ -2,6 +2,12 @@
 # Usage: vdl [LINK]
 vdl()
 {
+  # Check that ffmpeg and ffprobe are available.
+  if [[ "$(ffmpeg -version)" = "" ]] || [[ "$(ffprobe -version)" = "" ]]; then
+    echo -e "${color_red}ffmpeg and ffprobe are required.${color_default}"
+    return 2
+  fi
+
   local target="${@}" # What to download/update.
 
   # If no [LINK] provided, try to read from `src.txt`.