The Database page should display an error when connection is invalid

[pavish]

Description

• Add an invalid connection (eg., wrong host, port). Notice that the connection gets added.
• Open the database page. eg., http://localhost:8000/db/57/.
• Notice that the page displays Schemas (0), instead of displaying an error to the user.
• This is because the schemas endpoint filters by connection_id /api/db/v0/schemas/?connection_id=57, and returns: {"count":0,"results":[]}.

Expected behavior

• The database page should show an error mentioning that the connection failed.
• This might require both frontend and backend changes.

[Anish9901]

This seems to be by design, please read the issue description for #3348 and #3348 (comment)

I think we need more discussions on this behaviour.

[seancolsen]

We discussed this in a maintainers meeting today.

• Brent: We have an architectural problem causing this bug. This is why we see "Schemas (0)". There are 0 schema models associated with that connections model.
• Sean: is it possible to connect to a DB successfully without seeing any schemas? — yes. There are even some scenarios in which you can't see the public schema.
• Brent: This behavior is going to be completely changed when we don't have a schema model. In the new architecture this will be different.
• This is not a release-blocker for v0.1.4 though, so we'll move it into a later milestone.