opts.IPVal returns an error on incorrect input

Signed-off-by: Solomon Hykes <solomon@docker.com>

opts/ip.go

@@ -1,6 +1,7 @@
 package opts
 
 import (
+	"fmt"
 	"net"
 )
 
@@ -17,8 +18,10 @@ func NewIpOpt(ref *net.IP, defaultVal string) *IpOpt {
 }
 
 func (o *IpOpt) Set(val string) error {
-	// FIXME: return a parse error if the value is not a valid IP?
-	// We are not changing this now to preserve behavior while refactoring.
+	ip := net.ParseIP(val)
+	if ip == nil {
+		return fmt.Errorf("incorrect IP format")
+	}
 	(*o.IP) = net.ParseIP(val)
 	return nil
 }