QLS: give all actions errors

Instead of adding `Maybe Errors` to each constructor, which would probably lead
to quite a bit of duplicated code, I opted to add a single top-level constructor
that contains a `Maybe Errors` and an `Action'`. This `Action'` is a new data
type that contains the interesting constructors, like `New` and `Updates`.

As a result of this change, there quite a few mechanical changes, but not much
of it is interesting. However, having made this change, we are now ready to
enable fault injection for all the actions whenever we want to.

Author: jorisdral