Handle errors when eg, no current object is returned from API

[benfoley]

Most of the UI now blocks progress by disabling next/action buttons and dropzones unless there is a selected {dataset|pron_dict|model}, but we aren't actually catching the notifications that the API sends back for "no current thing".

To implement this, check for the status code in the response the action creators receive, and do make a new action eg for CONFLICTING_NAME.