cleanup DownloadTask reference in episode when cancelled

otherwise partial downloads cancelled on restart can't be
downloaded again until gPodder restart. Because they were
not run from a DownloadQueueWorker, which would have called
recycle() after run().

src/gpodder/download.py

@@ -759,6 +759,7 @@ class DownloadTask(object):
             util.delete_file(self.tempname)
             self.progress = 0.0
             self.speed = 0.0
+            self.recycle()
             return False
 
         # We only start this download if its status is "downloading"