Dashboard: When dashboard is not found show message instead of empty page

[polibb]

What is this feature?
Adds a view for Not Found dashboard.

Why do we need this feature?
Because Not Found dashboard is treated the same as an empty one right now.

Special notes for your reviewer:

I think we can also stop the initialisation of the dashboard here: initDashboard#L161 by sending a dispatch(dashboardInitFailed({ message: 'Not found', error: 'Not found' })) and render the Failed page. Thoughts?

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.
• There are no known compatibility issues with older supported versions of Grafana, or plugins.
• It passes the Hosted Grafana feature readiness review for observability, scalability, performance, and security.

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[dprokop]

Can we reuse the view from https://github.com/grafana/grafana/blob/dashboard/not-found-page/public/app/core/components/ErrorPage/ErrorPage.tsx#L8 for consistency?

Other than that this looks good.

[polibb]

Can we reuse the view from https://github.com/grafana/grafana/blob/dashboard/not-found-page/public/app/core/components/ErrorPage/ErrorPage.tsx#L8 for consistency?

Yes, and I like the fact that it looks less like a dashboard you can do stuff on 😁.