patch 9.0.0389: crash when 'tagfunc' closes the window

Problem:    Crash when 'tagfunc' closes the window.
Solution:   Bail out when the window was closed.
diff --git a/src/errors.h b/src/errors.h
index ee75d07..44eb9db 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3324,3 +3324,5 @@
 EXTERN char e_non_null_list_required_for_argument_nr[]
 	INIT(= N_("E1298: Non-NULL List required for argument %d"));
 #endif
+EXTERN char e_window_unexpectedly_close_while_searching_for_tags[]
+	INIT(= N_("E1299: Window unexpectedly closed while searching for tags"));