Handle transfer job being canceled

Set KilledJobError to indicate it was canceled by the user
to avoid a bogus "finished" notification.

Sadly, fileTransferCanceled has been removed from libQuotient
so this lambda botch checking transfer status needs to be done.

src/filetransferpseudojob.h

@@ -38,6 +38,11 @@ public:
      */
     void fileTransferFailed(const QString &id, const QString &errorMessage = {});
 
+    /**
+     * @brief Set the file transfer as canceled.
+     */
+    void fileTransferCanceled(const QString &id);
+
     /**
      * @brief Start the file transfer.
      */