tc: fix a misleading failure

tc: fix a misleading failure

Before this patch:

 # ./tc/tc actions add action drop index 11
 RTNETLINK answers: File exists
 We have an error talking to the kernel
 Command "(null)" is unknown, try "tc actions help".

After this patch:

 # ./tc/tc actions add action drop index 11
 RTNETLINK answers: File exists
 We have an error talking to the kernel

Cc: Stephen Hemminger <shemming@brocade.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>