Clean up controller by just throwing exceptions and rely on the exception filter.

[bobend]

eg. change

throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Machinename must be specified"));

to

throw new ArgumentNullException("Machinename must be specified",nameof(machineName));

[bobend]

This will also make the unittests more readable.

[mchadrdk]

Initial cleanup in 7304156

Changing the rest of the HttpResponseExceptions would take away the friendly error message from ModelState validation