Add middleware.NextOrFailure (#462)

This checks if the next middleware to be called is nil, and if so returns
ServerFailure and an error. This makes the next calling more robust and
saves some lines of code.

Also prefix the error with the name of the middleware to aid in
debugging.

middleware/middleware.go

@@ -2,6 +2,7 @@
 package middleware
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/miekg/dns"
@@ -65,5 +66,15 @@ func (f HandlerFunc) Name() string { return "handlerfunc" }
 // Error returns err with 'middleware/name: ' prefixed to it.
 func Error(name string, err error) error { return fmt.Errorf("%s/%s: %s", "middleware", name, err) }
 
+// NextOrFailure calls next.ServeDNS when next is not nill, otherwise it will return, a ServerFailure
+// and a nil error.
+func NextOrFailure(name string, next Handler, ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	if next != nil {
+		return next.ServeDNS(ctx, w, r)
+	}
+
+	return dns.RcodeServerFailure, Error(name, errors.New("no next middleware found"))
+}
+
 // Namespace is the namespace used for the metrics.
 const Namespace = "coredns"