Optimize the scene unload logic.

优化场景卸载逻辑，在加载新的主场景的时候自动卸载已经加载的所有场景。

Assets/YooAsset/Runtime/AssetSystem/Operations/InstantiateOperation.cs

@@ -48,12 +48,15 @@ namespace YooAsset
 					_steps = ESteps.Done;
 					Status = EOperationStatus.Failed;
 					Error = $"{nameof(AssetOperationHandle)} is invalid.";
+					return;
 				}
+
 				if (_handle.AssetObject == null)
 				{
 					_steps = ESteps.Done;
 					Status = EOperationStatus.Failed;
 					Error = $"{nameof(AssetOperationHandle.AssetObject)} is null.";
+					return;
 				}
 
 				if(_setPositionRotation)

Assets/YooAsset/Runtime/AssetSystem/Operations/UnloadSceneOperation.cs

@@ -12,7 +12,6 @@ namespace YooAsset
 		{
 			Normal,
 			Error,
-			Skip,
 		}
 		private enum ESteps
 		{
@@ -40,10 +39,6 @@ namespace YooAsset
 			}
 		}
 
-		internal UnloadSceneOperation()
-		{
-			_flag = EFlag.Skip;
-		}
 		internal UnloadSceneOperation(string error)
 		{
 			_flag = EFlag.Error;
@@ -60,11 +55,6 @@ namespace YooAsset
 			{
 				_steps = ESteps.UnLoad;
 			}
-			else if (_flag == EFlag.Skip)
-			{
-				_steps = ESteps.Done;
-				Status = EOperationStatus.Succeed;
-			}
 			else if (_flag == EFlag.Error)
 			{
 				_steps = ESteps.Done;