Parse osu-web error text to present invalid items to the user in a more approachable format

osu.Game/Screens/OnlinePlay/Playlists/PlaylistsRoomSettingsOverlay.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Specialized;
+using System.Linq;
 using Humanizer;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
@@ -349,9 +350,31 @@ namespace osu.Game.Screens.OnlinePlay.Playlists
 
             private void onError(string text)
             {
-                ErrorText.Text = text;
+                // see https://github.com/ppy/osu-web/blob/2c97aaeb64fb4ed97c747d8383a35b30f57428c7/app/Models/Multiplayer/PlaylistItem.php#L48.
-                ErrorText.FadeIn(50);
+                const string not_found_prefix = "beatmaps not found:";
 
+                if (text.StartsWith(not_found_prefix, StringComparison.Ordinal))
+                {
+                    ErrorText.Text = "One or more beatmap was not available online. Please remove or replaced the highlighted items.";
+
+                    int[] invalidBeatmapIDs = text
+                                              .Substring(not_found_prefix.Length + 1)
+                                              .Split(", ")
+                                              .Select(int.Parse)
+                                              .ToArray();
+
+                    foreach (var item in Playlist)
+                    {
+                        if (invalidBeatmapIDs.Contains(item.BeatmapID))
+                            item.MarkInvalid();
+                    }
+                }
+                else
+                {
+                    ErrorText.Text = text;
+                }
+
+                ErrorText.FadeIn(50);
                 loadingLayer.Hide();
             }
         }