Zombie group on entity creation if an exception is thrown

As @stoewer discovered during pr #530: When we create a new entity (e.g. `BlockHDF5`), the parent (e.g. `FileHDF5`) creates the group, then passes this group to the `BlockHDF5` constructor which initialises it (sets attributes, etc.). In the (unlikely, but not impossible) case that an exception gets thrown the group is not clean up after.

Review PR #530 and trace the entity-creation path involving FileHDF5 and BlockHDF5. Focus on the handoff of the newly created group to the constructor and determine how an exception during initialization is handled. Done means a failed construction does not leave the newly created group behind.

Written by the indexing model from the issue text.