Adds ResultOperation.localizedFailure

AltStore/Operations/Operation.swift

@@ -13,6 +13,9 @@ class ResultOperation<ResultType>: Operation
 {
     var resultHandler: ((Result<ResultType, Error>) -> Void)?
 
+    // Should only be set by subclasses
+    var localizedFailure: String?
+
     @available(*, unavailable)
     override func finish()
     {
@@ -23,14 +26,18 @@ class ResultOperation<ResultType>: Operation
     {
         guard !self.isFinished else { return }
 
+        var result = result
+
         if self.isCancelled
         {
-            self.resultHandler?(.failure(OperationError.cancelled))
+            result = .failure(OperationError.cancelled)
         }
-        else
+        else if case .failure(let nsError as NSError) = result, let localizedFailure, nsError.localizedFailure == nil {
-        {
+            // Error doesn't have its own localizedFailure, so we give it the Operation's (if it exists)
-            self.resultHandler?(result)
+            let error = nsError.withLocalizedFailure(localizedFailure)
+            result = .failure(error)
         }
+        self.resultHandler?(result)
 
         super.finish()
     }