Clearer error reporting from request handlers

[brandoniles]

Throwing exceptions "across the wire" is probably not the best error reporting strategy, but it would be nice to have some cleaner answer for this.

Right now the client handler code needs to do something like:

if (value == null) {
    err = new NotFoundError(key);
}
return new Response.Builder<>(new KeyValue.getValue_result(value, err), request.getEndpoint(), ResponseCode.OK) ...

Clients have to set the response code in parallel to the error in the response. It's also unclear whether the value is transmitted along with err when err is non-null (I think it's not). Some applications may have a partial response that can be used in addition to the reported error.