dgp	Sometimes I mistype a tag name durin `fossil open`:
dgp	fossil open core-8-5-brunch
dgp	It fails, gives me a good message.
dgp	So I try again, taking care to type it right, and it fails because I can't `fossil open` into a directory where there's already been a `fossil open` -- even though it failed.
dgp	My simple minded fix would be to delete the _FOSSIL_ file when a `fossil open` fails, but dunno what implications that has.