engine: `relax: true` creates an error

[sergkudinov]

Action returns an error when metadata: relax: true and error is not thrown in action.

To reproduce

run:

{error} = await nikita.call metadata: relax: true, -> true
console.log error

It returns an error:

TypeError: prom.then is not a function
    at /path/to/node-nikita/packages/engine/src/metadata/relax.coffee:27:14