this issue is now fixed. report issues to support site instead of here, so I will notified if there are any issues.

Thanks. I think we can't modify the error page or inject css to it though I can replace it with a custom page, this will cause issues with any such addon.
Alternatives I can think of is change title of tab (not possible in this api), create a icon in addon bar (still takes some valuable space), or check if it possible to change favicon of page.