Missing layers shouldn't stop rendering

[csytsma]

**A clear and concise description of what the problem is.
When viewing the base map, if there is a problem with a layer, such as it can't be found, or error with the SQL, the map stops rendering. A single layer will completely stop the workflow.

Describe the solution or feature that you'd like
The rendering of the map should gracefully continue even if a layer can't be rendered.

[Komzpa]

This means the error gets unnoticed and it takes even more time to find. Imagine database shutdown mid-export that will break half of exported map that will get published but be plain wrong.

[csytsma]

During the initial map rendor (non-export), it shouldn't go unreported. It should still display a modal error, and perhaps flag the Layer and the Layer Drawer that there is a Layer with an error. During an Export, it should stop if there is a problem with a Layer.  Ideally these could be handled differently.