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>
diff --git a/ip/link_vti.c b/ip/link_vti.c
index 77a7482..6274c83 100644
--- a/ip/link_vti.c
+++ b/ip/link_vti.c
@@ -171,8 +171,11 @@
 		} 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++;