Remove unused delegate, use model name in notifications, add more xmldoc

- Applies a `class` constraint to the generic type in `IModelManager`
- Add xmldoc

osu.Game/Database/DownloadableArchiveModelManager.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using Humanizer;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Game.Online.API;
@@ -112,8 +113,7 @@ namespace osu.Game.Database
                 if (error is OperationCanceledException) return;
 
                 notification.State = ProgressNotificationState.Cancelled;
-                // TODO: maybe implement a Name for every model that we can use in this message?
-                Logger.Error(error, "Download failed!");
+                Logger.Error(error, $"{HumanisedModelName.Titleize()} download failed!");
                 currentDownloads.Remove(request);
             };