Repo policy to warn on depexts without post-messages { failure }

[dsheets]

It is generally nice to have a human-readable message in the case of package installation failure when a package relies upon depexts. More than the usual "use opam depext" message, this message can specify information like the version of a depext required.

[AltGr]

Ah, we actually have this warning already (nb. 38), but it was disabled since more friendly messages were added to opam in case of depexts.