python: fix json encoder should handle Exception

python: fix json encoder should handle Exception

This happens if --json is used and a CommandError is raised, so will affect other commands too where --json is used.

This happens in the print_json_status method.

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>