`onResult` hook for _all_ operation results (including parsing and validation errors)

[enisdenjo]

Currently, the onExecute and onExecuteDone hooks are invoked exclusively if execution occurs. If errors happen beforehand (like with parsing or validation issues), onExecute will not be invoked and the final result cannot be manipulated.

Introduce a new onResult hook allowing manipulation of the final result regardless of what happened.

Necessary for #1490

[enisdenjo]

Actually, due to the envelop relying on the user to construct the final result on parsing and validation errors, this hook cannot be implemented.

We'd need an ultimate "parse, validate and execute/subscribe" function (accompanied with an appropriate hook) to achieve this.