[AltStore] Improves app installation cancellation/progress tracking

AltStore/Operations/AppOperationContext.swift

@@ -25,6 +25,8 @@ class AppOperationContext
     var resignedFileURL: URL?
     var connection: NWConnection?
     
+    var isFinished = false
+    
     var error: Error? {
         get {
             return _error ?? self.group.error

AltStore/Operations/Operation.swift

@@ -23,7 +23,14 @@ class ResultOperation<ResultType>: Operation
     {
         guard !self.isFinished else { return }
         
-        self.resultHandler?(result)
+        if self.isCancelled
+        {
+            self.resultHandler?(.failure(OperationError.cancelled))
+        }
+        else
+        {
+            self.resultHandler?(result)
+        }
         
         super.finish()
     }