Generate server-side code for error return types

[gkanz]

Currently, return error <: ErrorType is handled by svc_client.sysl, but not by svc_handler.sysl. Errors returned by the generated server are forced to be of type:

type HTTPError struct {
	HTTPCode    int    `json:"-"`
	Code        string `json:"code,omitempty"`
	Description string `json:"description,omitempty"`
}

svc_handler.sysl should be updated to allow sysl-specified error types to be returned by the server.