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

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

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

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

squash! dabbad: return error code in 'interface coalesce'.

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

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