fix incorrect ErrConnectFailed comparison

Signed-off-by: Reficul <xuzhenglun@gmail.com>

errors.go

@@ -1,18 +1,34 @@
 package client
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/docker/docker/api/types/versions"
+	"github.com/pkg/errors"
 )
 
-// ErrConnectionFailed is an error raised when the connection between the client and the server failed.
-var ErrConnectionFailed = errors.New("Cannot connect to the Docker daemon. Is the docker daemon running on this host?")
+// errConnectionFailed implements an error returned when connection failed.
+type errConnectionFailed struct {
+	host string
+}
+
+// Error returns a string representation of an errConnectionFailed
+func (err errConnectionFailed) Error() string {
+	if err.host == "" {
+		return "Cannot connect to the Docker daemon. Is the docker daemon running on this host?"
+	}
+	return fmt.Sprintf("Cannot connect to the Docker daemon at %s. Is the docker daemon running?", err.host)
+}
+
+// IsErrConnectionFailed returns true if the error is caused by connection failed.
+func IsErrConnectionFailed(err error) bool {
+	_, ok := errors.Cause(err).(errConnectionFailed)
+	return ok
+}
 
 // ErrorConnectionFailed returns an error with host in the error message when connection to docker daemon failed.
 func ErrorConnectionFailed(host string) error {
-	return fmt.Errorf("Cannot connect to the Docker daemon at %s. Is the docker daemon running?", host)
+	return errConnectionFailed{host: host}
 }
 
 type notFound interface {