Don't discard error message on sending response

Error messages are always discarded and it makes diagnostic difficult
when error occurs.

Before:
$ dnet network create -d overlay ov1
error : ""
$ docker network create -d overlay ov1
Error response from daemon: ""

After:
$ dnet network create -d overlay ov1
error : "no datastore configured. cannot obtain vxlan id"
$ docker network create -d overlay ov1
Error response from daemon: "no datastore configured. cannot obtain vxlan id"

Breakage caused by  7b8bb3e

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>

api/api.go

@@ -168,6 +168,10 @@ func makeHandler(ctrl libnetwork.NetworkController, fct processor) http.HandlerF
 		}
 
 		res, rsp := fct(ctrl, mux.Vars(req), body)
+		if !rsp.isOK() {
+			http.Error(w, rsp.Status, rsp.StatusCode)
+			return
+		}
 		if res != nil {
 			writeJSON(w, rsp.StatusCode, res)
 		}