Disallow tab=None with objreg.get(scope='tab').

This would've made it a lot easier to detect #563.

qutebrowser/browser/commands.py

@@ -529,7 +529,7 @@ class CommandDispatcher:
         url = self._current_url()
         if frame is None:
             raise cmdexc.CommandError("No frame focused!")
-        hintmanager = objreg.get('hintmanager', scope='tab')
+        hintmanager = objreg.get('hintmanager', scope='tab', tab='current')
         if where == 'prev':
             hintmanager.follow_prevnext(frame, url, prev=True, tab=tab,
                                         background=bg, window=window)

qutebrowser/utils/objreg.py

@@ -144,7 +144,7 @@ window_registry = ObjectRegistry()
 def _get_tab_registry(win_id, tab_id):
     """Get the registry of a tab."""
     if tab_id is None:
-        tab_id = 'current'
+        raise ValueError("Got tab_id None (win_id {})".format(win_id))
     if tab_id == 'current' and win_id is None:
         app = get('app')
         window = app.activeWindow()