If config.xml and/or OMERODIR are required return an error if missing

[manics]

See https://github.com/ome/omero-web/issues/58 Automatically switching to a different directory if config.xml is missing creates confusion.

It would be clearer if a missing OMERODIR and/or config.xml threw an error so a sysadmin knows something is incorrectly configured.

Presumably this only affects servers (OMERO.server and OMERO.web), and not clients so there should be no harm to usability of the CLI