Add robust error handling.

.local/bin/channelrefresh

@@ -1,10 +1,21 @@
 #!/bin/dash
 
-sleep 16
+while ! ping -c 1 9.9.9.9 > /dev/null 2>&1; do sleep 0.5; done
 
 DATA_DIR="$HOME/.cache/youtube_channels"
 CHANNEL_LIST="$HOME/.local/share/channels.txt"
-mkdir -p "$DATA_DIR"
+mkdir -p "$DATA_DIR" && touch "$CHANNEL_LIST"
+
+error_handling() {
+    [ -s "$CHANNEL_LIST" ] || {
+	    notify-send "You don't have any channels in 'channels.txt'."
+	    exit 1
+    }
+    grep -q "^.*=https://www.youtube.com/@[[:alnum:]]*/videos$" "$CHANNEL_LIST" || {
+	    notify-send "'channels.txt' formatting is wrong."
+	    exit 1
+    }
+}
 
 compare_data() {
   local channel_name="$1"
@@ -47,4 +58,5 @@ update_all_channels() {
   done < "$CHANNEL_LIST"
 }
 
+error_handling
 update_all_channels