segtree: set proper error cause on existing elements

Adding new elements result in a confusing "Success" error message.

 # nft add element x y { 0-3 }
 [...]
 Error: Could not process rule: Success
 add element x y { 0-3 }
 ^^^^^^^^^^^^^^^^^^^^^^^^

after this patch, this reports:

 Error: Could not process rule: File exists
 add element x y { 0-3 }
 ^^^^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

0 files changed, 0 insertions, 0 deletions