Add Unwrap error to custom error types

Signed-off-by: Derek McGowan <[email protected]>

Author: dmcgowan

agent/exec/errors.go

@@ -64,6 +64,7 @@ type temporary struct {
 	error
 }
 
+func (t temporary) Unwrap() error   { return t.error }
 func (t temporary) Cause() error    { return t.error }
 func (t temporary) Temporary() bool { return true }
 

swarmd/dockerexec/controller.go

@@ -16,11 +16,12 @@ import (
 	engineapi "github.com/docker/docker/client"
 	"github.com/docker/go-connections/nat"
 	gogotypes "github.com/gogo/protobuf/types"
+	"github.com/pkg/errors"
+	"golang.org/x/time/rate"
+
 	"github.com/moby/swarmkit/v2/agent/exec"
 	"github.com/moby/swarmkit/v2/api"
 	"github.com/moby/swarmkit/v2/log"
-	"github.com/pkg/errors"
-	"golang.org/x/time/rate"
 )
 
 // controller implements agent.Controller against docker's API.
@@ -597,6 +598,10 @@ func (e *exitError) Cause() error {
 	return e.cause
 }
 
+func (e *exitError) Unwrap() error {
+	return e.cause
+}
+
 func makeExitError(ctnr types.ContainerJSON) error {
 	if ctnr.State.ExitCode != 0 {
 		var cause error