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/whoami/whoami_test.go

@@ -3,7 +3,6 @@ package whoami
 import (
 	"testing"
 
-	"github.com/miekg/coredns/middleware"
 	"github.com/miekg/coredns/middleware/pkg/dnsrecorder"
 	"github.com/miekg/coredns/middleware/test"
 
@@ -15,7 +14,6 @@ func TestWhoami(t *testing.T) {
 	wh := Whoami{}
 
 	tests := []struct {
-		next          middleware.Handler
 		qname         string
 		qtype         uint16
 		expectedCode  int
@@ -23,7 +21,6 @@ func TestWhoami(t *testing.T) {
 		expectedErr   error
 	}{
 		{
-			next:          test.NextHandler(dns.RcodeSuccess, nil),
 			qname:         "example.org",
 			qtype:         dns.TypeA,
 			expectedCode:  dns.RcodeSuccess,
@@ -35,7 +32,6 @@ func TestWhoami(t *testing.T) {
 	ctx := context.TODO()
 
 	for i, tc := range tests {
-		wh.Next = tc.next
 		req := new(dns.Msg)
 		req.SetQuestion(dns.Fqdn(tc.qname), tc.qtype)