PG: _scan_list can now handle EIO on read, stat, omap_get_header

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: rename debug_delete_obj to debug_obj_on_delete

This should make the method intent less confusing.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG::_scan_list: assert if error is neither -EIO nor -ENOENT

Signed-off-by: Samuel Just <sam.just@inktank.com>