Return a `Msg of string` instead of a simple `string` when we got an error

[dinosaure]

In keys.ml, we returns a simple string which does not compose well with rresult when this package is available (specially with mirage). It should be nice to return an Msg of string, and, by this way, be able to use for instance Rresult.R.failwith_error_msg.

[hannesm]

I agree that this would be very fine -- though I'd also avoid a rresult dependency in this package (it is >= 4.08.0 anyways, which has a decent Result module).