Centralize error toast notification

[alexashley]

Comes out of #164:

On the backend of rode-ui, we were able to proxy status codes with less explicit handling; however, adding a check for showing an unauthorized message meant lots of tedious checks that look like this on the front-end:

if (response.status === FORBIDDEN) {
   return showError(UNAUTHORIZED_ERROR_MESSAGE);
}

We're using fetch directly in a lot of places, might be desirable to pull that out into a single spot. There's a useFetch hook that might be the right spot, but all of the existing call sites would have to be updated.

This would also apply for showing a generic error message if the call fails for another reason.

[alexashley]

This is also an issue when the user doesn't have a role that Rode recognizes -- passing this error along would be handy:

{"code":16,"message":"missing roles claim","details":[]}