Indicate if set_active_tab() worked or not

kitty/boss.py

@@ -846,7 +846,8 @@ class Boss:
     def set_active_tab(self, tab):
         tm = self.active_tab_manager
         if tm is not None:
-            tm.set_active_tab(tab)
+            return tm.set_active_tab(tab)
+        return False
 
     def next_tab(self):
         tm = self.active_tab_manager

kitty/tabs.py

@@ -478,8 +478,9 @@ class TabManager:  # {{{
         try:
             idx = self.tabs.index(tab)
         except Exception:
-            return
+            return False
         self.set_active_tab_idx(idx)
+        return True
 
     def next_tab(self, delta=1):
         if len(self.tabs) > 1: