When receiving error responses from our APIs, this gives us better toast
messages.

The crux of this issue was that we had no steady state when the server
returned a non-API error (!isApiError), as it would if the server was
throwing 500s or just generally misbehaving.

The solution, then, is to addresse these non-API errors in our
underlying useListsIndex and useListsPrivileges hooks.

This also refactors those hooks to:

* collapse multiple error states into one (that's all we currently need)
* use useAppToasts for better UI

TODO: I don't think I need the changes in useListsConfig's useEffect.

The only task in this hook is our readPrivileges task right now, so I'm
shortening the variable until we have a need to disambiguate it further.

If the index hook has an error needsIndex will not be true.

Our isApiError predicate does not work for errors coming back from
Kibana platform itself, e.g. for a request payload error.

I've added a separate predicate for that case, isKibanaError, and then a
wrapping isAppError predicate since most of our use cases just care
about error.body.message, which is common to both.

This fixes two issues:

* toast appears above modal overlay
* Error message from response is now presented in the toast

Because of the way some of the exception modal's hooks are written, a
change to one of its callbacks means that the request will be canceled.

Because the toasts service exports instance methods, the context within
the function (and thus the function itself) can change leading to a
mutable ref.

Because we don't want/need this behavior, we store our exported
functions in refs to 'freeze' them for react.

With our bound functions, we should now be able to declare e.g.
`toast.addError` as a dependency, however react cannot determine that it
is bound (and thus that toast.addError() is equivalent to addError()),
and so we must destructure our functions in order to use them as
dependencies.

This fixes the z-index issue between modals and toasts.

These tests now call out to the Notifications service (in a context)
instead of our redux implementation.