network: Add error message if iw couldn't be found

When iw is not installed on the host, lxc fails to move wireless devices
into a container. Unfortunately there is no real error message yet
(other than that it didn't work), so its quite unobvious what causes
this. This commit adds a error message that clearly states the
application iw is missing.

Signed-off-by: Maximilian Blenk <blenkmax@gmail.com>

src/lxc/network.c

@@ -1237,8 +1237,10 @@ int lxc_netdev_move_wlan(char *physname, const char *ifname, pid_t pid,
 	 * the 80211 module, so for now just call out to iw.
 	 */
 	cmd = on_path("iw", NULL);
-	if (!cmd)
+	if (!cmd) {
+		ERROR("Couldn't find the application iw in PATH");
 		return -1;
+	}
 
 	fpid = fork();
 	if (fpid < 0)