When callback of one config line fail, we'd better give the exactly
info to help users locate the misconfig.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>