Add a stop-gap solution for AssertionError when retrying downloads

See #3847

doc/changelog.asciidoc

@@ -90,6 +90,7 @@ Fixed
 - `@match` in Greasemonkey scripts now more closely matches the proper pattern
   syntax.
 - Searching via `/` or `?` now doesn't handle any characters in a special way.
+- Fixed crash when trying to retry some failed downloads on QtWebEngine.
 
 v1.2.1
 ------

qutebrowser/browser/webengine/webenginedownloads.py

@@ -101,7 +101,11 @@ class DownloadItem(downloads.AbstractDownloadItem):
 
     def retry(self):
         state = self._qt_item.state()
-        assert state == QWebEngineDownloadItem.DownloadInterrupted, state
+        if state != QWebEngineDownloadItem.DownloadInterrupted:
+            log.downloads.warning(
+                "Trying to retry download in state {}".format(
+                    debug.qenum_key(QWebEngineDownloadItem, state)))
+            return
 
         try:
             self._qt_item.resume()