Use a typed error for consumers of libmachine

Signed-off-by: David Gageot <david@gageot.net>

libmachine/host/host.go

@@ -2,14 +2,13 @@ package host
 
 import (
 	"errors"
-	"fmt"
 	"regexp"
-	"strings"
 
 	"github.com/docker/machine/libmachine/auth"
 	"github.com/docker/machine/libmachine/drivers"
 	"github.com/docker/machine/libmachine/engine"
 	"github.com/docker/machine/libmachine/log"
+	"github.com/docker/machine/libmachine/mcnerror"
 	"github.com/docker/machine/libmachine/mcnutils"
 	"github.com/docker/machine/libmachine/provision"
 	"github.com/docker/machine/libmachine/provision/pkgaction"
@@ -94,7 +93,10 @@ func (creator *StandardSSHClientCreator) CreateSSHClient(d drivers.Driver) (ssh.
 
 func (h *Host) runActionForState(action func() error, desiredState state.State) error {
 	if drivers.MachineInState(h.Driver, desiredState)() {
-		return fmt.Errorf("Machine %q is already %s.", h.Name, strings.ToLower(desiredState.String()))
+		return mcnerror.ErrHostAlreadyInState{
+			Name:  h.Name,
+			State: desiredState,
+		}
 	}
 
 	if err := action(); err != nil {

libmachine/mcnerror/errors.go

@@ -3,6 +3,9 @@ package mcnerror
 import (
 	"errors"
 	"fmt"
+	"strings"
+
+	"github.com/docker/machine/libmachine/state"
 )
 
 var (
@@ -32,3 +35,12 @@ type ErrDuringPreCreate struct {
 func (e ErrDuringPreCreate) Error() string {
 	return fmt.Sprintf("Error with pre-create check: %q", e.Cause)
 }
+
+type ErrHostAlreadyInState struct {
+	Name  string
+	State state.State
+}
+
+func (e ErrHostAlreadyInState) Error() string {
+	return fmt.Sprintf("Machine %q is already %s.", e.Name, strings.ToLower(e.State.String()))
+}