Raise an error if repository is blank

Otherwise the error is not friendly: `/Users/kir/Projects/opensource/rom/lib/rom/schema/builder.rb:57:in `fetch': key not found: nil (KeyError)`
We could change this check to `unless definition.repository` because we simply expect it to be defined. If it's not a valid repo name it'll raise anyway. One thing we could consider doing is having a custom repository index class that raises a meaningful error when something is trying to fetch a repo that is not registered. It's a common pattern in ROM so it might even make sense to have a generic registry class that knows about type of objects that it has. We store relations, repositories and mappers currently.