UPDATE: I worked it out. But for anyone else having the same problem, the solution is that the error details should not be returned as a HTTP status but instead should be detailed in the response.error object.
When reading a folder I am attempting to return a specific status code from the controller under certain circumstances. For example, if the current user does not have permission to read the selected folder I am returning a 403 code indicating "Not Allowed". I would like to catch this in the FileManager.failure event and display an appropriate error message on the client.
My code is clearly returning the correct status code & message but these details are not being passed to the failure event. The args property passed to the failure event always contains a 404 status with a "NetworkError..." error message.
The server code used to return the error message is:
Return Content(Net.HttpStatusCode.Forbidden, deniedEx.Message)
Please let me know if there is a different, preferred method to achieve this.