lint(errorlint): handle wrapped errors

Enable errorlint and preserve wrapped error chains so runtime checks
and tests classify failures correctly. This also makes Route53
surface insert failures instead of silently dropping them.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/forward/forward.go

@@ -169,7 +169,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		for {
 			ret, err = proxy.Connect(ctx, state, opts)
 
-			if err == proxyPkg.ErrCachedClosed { // Remote side closed conn, can only happen with TCP.
+			if errors.Is(err, proxyPkg.ErrCachedClosed) { // Remote side closed conn, can only happen with TCP.
 				continue
 			}
 			// Retry with TCP if truncated and prefer_udp configured.

plugin/forward/health_test.go

@@ -2,6 +2,7 @@ package forward
 
 import (
 	"context"
+	"errors"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -343,8 +344,8 @@ func TestAllUpstreamsDown(t *testing.T) {
 		t.Errorf("Expected Response code: %d, Got: %d", dns.RcodeServerFailure, resp)
 	}
 
-	if err != ErrNoHealthy {
-		t.Errorf("Expected error message: no healthy proxies, Got: %s", err.Error())
+	if !errors.Is(err, ErrNoHealthy) {
+		t.Errorf("Expected error message: no healthy proxies, Got: %v", err)
 	}
 
 	q1 := atomic.LoadUint32(&qs)
@@ -358,7 +359,7 @@ func TestAllUpstreamsDown(t *testing.T) {
 	req = new(dns.Msg)
 	req.SetQuestion("example.org.", dns.TypeA)
 	_, err = f.ServeDNS(context.TODO(), &test.ResponseWriter{}, req)
-	if err == ErrNoHealthy {
+	if errors.Is(err, ErrNoHealthy) {
 		t.Error("Unexpected error message: no healthy proxies")
 	}