Error out early when work dir does not exists and is expected to. (#171)

* Error out early when work dir does not exists and is expected to.

Auxilliary commands such as pgcopydb list progress expect the work directory
to have been created already. Check that early in the code.

* Fix copydb_init_workdir to specify if work dir should be created.

Then fix the callers and make most of them re-use a pre-existing directory
rather than create a default one and then use it.