Allow files missing on disk to be restored on beatmap import

Previously, in the rare case the database became out of sync with the disk store, it was impossible to feasibly repair a beatmap. Now reimporting checks each file exists on disk and adds it back if it doesn't.

osu.Game/Beatmaps/BeatmapManager.cs

@@ -429,6 +429,15 @@ namespace osu.Game.Beatmaps
             if (beatmapSet != null)
             {
                 Undelete(beatmapSet);
+
+                // ensure all files are present and accessible
+                foreach (var f in beatmapSet.Files)
+                {
+                    if (!storage.Exists(f.FileInfo.StoragePath))
+                        using (Stream s = reader.GetStream(f.Filename))
+                            files.Add(s, false);
+                }
+
                 return beatmapSet;
             }