Only support :debug-clear-ssl-errors with QtWebKit

qutebrowser/browser/commands.py

@@ -1992,7 +1992,7 @@ class CommandDispatcher:
                 tab.send_event(release_event)
 
     @cmdutils.register(instance='command-dispatcher', scope='window',
-                       debug=True)
+                       debug=True, backend=usertypes.Backend.QtWebKit)
     def debug_clear_ssl_errors(self):
         """Clear remembered SSL error answers."""
         self._current_widget().clear_ssl_errors()

qutebrowser/browser/webengine/webenginetab.py

@@ -569,7 +569,7 @@ class WebEngineTab(browsertab.AbstractTab):
         self._widget.setHtml(html, base_url)
 
     def clear_ssl_errors(self):
-        log.stub()
+        raise browsertab.UnsupportedOperationError
 
     @pyqtSlot()
     def _on_history_trigger(self):