pybind/rados.py: improve error output

Fixes: 7264

When failing to load librados, output the exception.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1163 from ceph/wip-7264

pybind/rados.py: improve error output

Reviewed-by: Dan Mick <dan.mick@inktank.com>