Generate errorResponse via built-in helper function to custom errors

[gr812b]

Related to #149

Currently we manually create an errorResponse object in the ExceptionMapper. This pattern worked when we were dealing with effectively random errors that could be thrown in any way. Now that we've converted to ensuring all errors are custom thrown ones, we should be able to generalize this to instead make use of a custom function. By adding a function to the base custom error, and letting all sub-errors inherit this method, all errors can be converted to an ErrorResponse in a clean manner.