do not exit silently when link is not found

When we create a tunnel on top of a link and the link specified
in cmdline doesn't exist, an error message should be shown.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

ip/iplink_vxlan.c

@@ -99,8 +99,11 @@ static int vxlan_parse_opt(struct link_util *lu, int argc, char **argv,
 		} else if (!matches(*argv, "dev")) {
 			NEXT_ARG();
 			link = if_nametoindex(*argv);
-			if (link == 0)
+			if (link == 0) {
+				fprintf(stderr, "Cannot find device \"%s\"\n",
+					*argv);
 				exit(-1);
+			}
 		} else if (!matches(*argv, "ttl") ||
 			   !matches(*argv, "hoplimit")) {
 			unsigned uval;

ip/iptunnel.c

@@ -250,8 +250,11 @@ static int parse_args(int argc, char **argv, int cmd, struct ip_tunnel_parm *p)
 
 	if (medium[0]) {
 		p->link = if_nametoindex(medium);
-		if (p->link == 0)
+		if (p->link == 0) {
+			fprintf(stderr, "Cannot find device \"%s\"\n",
+				medium);
 			return -1;
+		}
 	}
 
 	if (p->i_key == 0 && IN_MULTICAST(ntohl(p->iph.daddr))) {

ip/link_gre.c

@@ -207,8 +207,11 @@ get_failed:
 		} else if (!matches(*argv, "dev")) {
 			NEXT_ARG();
 			link = if_nametoindex(*argv);
-			if (link == 0)
+			if (link == 0) {
+				fprintf(stderr, "Cannot find device \"%s\"\n",
+					*argv);
 				exit(-1);
+			}
 		} else if (!matches(*argv, "ttl") ||
 			   !matches(*argv, "hoplimit")) {
 			unsigned uval;

ip/link_gre6.c

@@ -225,8 +225,11 @@ get_failed:
 		} else if (!matches(*argv, "dev")) {
 			NEXT_ARG();
 			link = if_nametoindex(*argv);
-			if (link == 0)
+			if (link == 0) {
+				fprintf(stderr, "Cannot find device \"%s\"\n",
+					*argv);
 				exit(-1);
+			}
 		} else if (!matches(*argv, "ttl") ||
 			   !matches(*argv, "hoplimit")) {
 			__u8 uval;

ip/link_vti.c

@@ -171,8 +171,11 @@ get_failed:
 		} else if (!matches(*argv, "dev")) {
 			NEXT_ARG();
 			link = if_nametoindex(*argv);
-			if (link == 0)
+			if (link == 0) {
+				fprintf(stderr, "Cannot find device \"%s\"\n",
+					*argv);
 				exit(-1);
+			}
 		} else
 			usage();
 		argc--; argv++;