Obnam: further tidying up

Goal Today I'm going to do more tidying up. In the near future I'm going to need begin work on actual backups, but I'll want the foundation to be strong and clean by then. Also, it's warm and I'm tired. Plan Change the client so that the LocalRepositoryError error type never used except for local repositories, in the obnam/src/bin/cmd/mod.rs module. Change the client command line parser to be able…