Currently we're raising an InvalidParamsError or ApiError to return a failure response.

@method def fruits(color): if color not in ("red", "orange", "yellow"): raise InvalidParamsError("No fruits of that colour")

I would prefer not to raise an exception for these, because it

1. uses exceptions to control flow
2. makes the @method a partial function
3. goes against the functional style of the rest of the library

Instead we should return an error response.