Mark almost-impossible check with a nocover pragma

This can only happen when running :download-open on a download created
by :adblock-update right now, which is a very small chance, so we can't
easily test for it.

qutebrowser/browser/webkit/downloads.py

@@ -533,7 +533,7 @@ class DownloadItem(QObject):
         filename = self._filename
         if filename is None:
             filename = getattr(self.fileobj, 'name', None)
-        if filename is None:
+        if filename is None:  # pragma: no cover
             log.downloads.error("No filename to open the download!")
             return