when a dashboard query variable's query fails, in some cases there's no info shown in the UI

when you have a dashboard-variable backed by a query, and the query-response arrives as a non-http-200 ajax-response, no visual representation is shown for this error. the reason is this: the error is created here: https://github.com/grafana/grafana/blob/8afab6a80fe91f6facdaf6ead7c9ac2abb2ceb8d/public/app/core/services/backend_srv.ts#L346-L353 and the visualisation is handled here: https://github.com/grafana/grafana/blob/8afab6a80fe91f6facdaf6ead7c9ac2abb2ceb8d/public/app/features/variables/query/actions.ts#L53 the problem is, the created `FetchError` is not an instance of `Error`, so it is not visualised. we should adjust `FetchError` to be an instance of `Error`.