Avoid reporting an import as successful when all beatmaps failed to import

osu.Game/Beatmaps/BeatmapImporter.cs

@@ -434,6 +434,9 @@ private List<BeatmapInfo> createBeatmapDifficulties(BeatmapSetInfo beatmapSet, R
                 }
             }
 
+            if (!beatmaps.Any())
+                throw new ArgumentException($"No valid beatmap files found in the beatmap archive.");
+
             return beatmaps;
         }
     }