Update requestAPI handler to allow full server response to be processed

[RRosio]

While the server may send detailed information about error messages, for status codes >= 400, the following check prevents the frontend from accessing that server response (the payload shows up in the dev console but the response object is null).

jupyter-fsspec/src/handler/handler.ts

Lines 41 to 43 in c2690fb

	if (!response.ok) {
	throw new ServerConnection.ResponseError(response, data.message || data);
	}

[RRosio]

This is related to #86 and was found when debugging #88.