subrepo: make subrepo.subrepo(<not a subrepo path>) fail Until now, when calling subrepo.subrepo with a path at which there is no subrepo, a "nullstate" tuple would be returned. However, this is not very useful (the tuple can't really be used for creating a subrepo), so we'd just as soon have the function just fail, and leave it up to the caller to decide what to do. The motivation for doing this now is to simplify the solution for (issue3056).