rpc: add error code in 'interface capabilities' msg.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
rpc: return error code on 'interface modify capabilities'.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabbad: return error code in 'interface capabilities'.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabba: support error code in 'interface capabilities'.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>