Tests needed:

   - Fail if cache_dir from previous run cannot be deleted
   - Fail if cache_dir cannot be created from current out_dir
   - transfer_type_explorers: Explorers are not transferred twice
   - transfer_type_explorers: No transfer tried if there are no type_explorers

   - does $require work?
   - $whatever should fail if there is no global explorer directory
   - emulator may only be called with __ as prefix - fail otherwise!

   - ensure paths returned by object include dot-cdist
   - ensure path of explorer of object returns correct path

Code fixes needed:

   - shutil, os.mkdir, etc. everywhere: catch/reraise exceptions correctly