Deal with context invalidation in the sidebar (notify user, reload?)

[fregante]

When the extension is reloaded, all pages stop receiving messages, including any sidebars currently on the page. This is not immediately visible and will result into failed retries of SIDEBAR_RENDER_PANELS towards the attached sidebar, which is visible but cannot receive messages.

Tasks

• [x] Notify user of context invalidation #4117
• [ ] Adapt ErrorBanner to work in the sidebar instead of using a notification
• [ ] When messaging the sidebar, detect this state and suggest reloading it? We should probably not automatically reload it to avoid user data loss (e.g. while filling a form)