Subst plugin doesn't work

The Subst plugin fails with:

> Loading plugin 'plugins/Subst.hs'...
>
> plugins/Subst.hs:11:1:
> Failed to load interface for ‘Control.Monad.CatchIO’
> Perhaps you meant
> Control.Monad.Catch (from exceptions-0.8.0.2)
> Control.Monad.Catch.Pure (from exceptions-0.8.0.2)
> gitit: Error loading plugin: plugins/Subst.hs

Start with plugins/Subst.hs at line 11 and inspect the import that fails while loading the plugin. Check the available exception interfaces named in the error, then verify that the Subst plugin loads without the reported Control.Monad.CatchIO failure.

Written by the indexing model from the issue text.