Config.get(...) should raise interface-datastore.errors.notFound

[AuHau]

I am not 100% sure about this, but while working on integrating the js-ipfs-repo-migrations I stumbled on a case where I have expected to ipfs-repo throw an interface-datastore.errors.notFound error when config.get() won't find the required key.

I now that interface-datastore.errors are related to datastores and this is only somehow related to it as ipfs-repo.config creates an abstraction layer on top of it. But it would be good to have some easy and clean way how to test for not found errors other than err.message.match('does not exist').

[AuHau]

Solved by #201 just not by throwing interface-datastore.errors.notFound but by throwing custom error from repo level.