Ensure frontend displays errors when getting 400 from the server

[r4victor]

The server returns 400 errors with the following body:

{
    "detail": [
        {
            "msg": "Backend aws has active instances. Delete instances before deleting the backend.",
            "code": "error"
        }
    ]
}

The frontend should by default display an error notification when getting such response from the server. Currently, 400 responses are ignored in many places.

For example, the frontend should show an error when deleting a backend fails (#2114 will make it an error to delete a backend with active instances).