mgr: return the error if set_config() fails

mgr: return the error if set_config() fails

in general, `ActivePyModules::set_config()` is called by mgr module when
serving user commands updating module, sometimes if the option is of the
wrong type or invalid value, monitor rejects this request sent by mgr,
but the error info is only logged in the logging message on mgr, but not
returned to user. in this change, `ceph_set_module_option()` and the
underlying methods are updated to return the error to the caller as an
python exception.

Signed-off-by: Kefu Chai <kchai@redhat.com>