Make better error reporting for client's call to rados.Rados

Signed-off-by: Derrick Schneider derrick.schneider@gmail.com
Merge pull request #2767 from derricks/more_verbose_error_output

Make better error reporting for client's call to rados.Rados

Reviewed-by: Loic Dachary <loic-201408@dachary.org>