Restore error type in FindNetwork

The error type libnetwork.ErrNoSuchNetwork is used in the controller
to retry the network creation as a managed network though the manager.
The change of the type was breaking the logic causing the network to
not being created anymore so that no new container on that network
was able to be launched
Added unit test

Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>

daemon/daemon_test.go

@@ -7,6 +7,7 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/docker/docker/api/errdefs"
 	containertypes "github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/container"
 	_ "github.com/docker/docker/pkg/discovery/memory"
@@ -17,6 +18,8 @@ import (
 	"github.com/docker/docker/volume/local"
 	"github.com/docker/docker/volume/store"
 	"github.com/docker/go-connections/nat"
+	"github.com/docker/libnetwork"
+	"github.com/pkg/errors"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -311,3 +314,12 @@ func TestValidateContainerIsolation(t *testing.T) {
 	_, err := d.verifyContainerSettings(runtime.GOOS, &containertypes.HostConfig{Isolation: containertypes.Isolation("invalid")}, nil, false)
 	assert.EqualError(t, err, "invalid isolation 'invalid' on "+runtime.GOOS)
 }
+
+func TestFindNetworkErrorType(t *testing.T) {
+	d := Daemon{}
+	_, err := d.FindNetwork("fakeNet")
+	_, ok := errors.Cause(err).(libnetwork.ErrNoSuchNetwork)
+	if !errdefs.IsNotFound(err) || !ok {
+		assert.Fail(t, "The FindNetwork method MUST always return an error that implements the NotFound interface and is ErrNoSuchNetwork")
+	}
+}