Review concept of optional dependencies

[FRosner]

Problem

Currently, optional dependencies only influence the way a command graph is built from an XML catalog. If an optional dependency is missing in the catalog, the parsing will continue.

We might want to remove this to require catalogs that are properly set up. It is also considerable to rethink the influence of failing optional dependencies during execution. Maybe we don't need to abort the execution in that case.

[FRosner]

@Husterknupp please include a comment with a reference to this ticket in the place you add this exceptional parsing (CommandMananger.of()) or something.