pybind: improve EnvironmentError output

Follow 70c7c9e600c5 ("pybind/rados.py: improve error output") and
improve EnvironmentError output for librbd and libcephfs too.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1170 from ceph/wip-pybind-enverr

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>