Add helpers to create errdef errors

Instead of having to create a bunch of custom error types that are doing
nothing but wrapping another error in sub-packages, use a common helper
to create errors of the requested type.

e.g. instead of re-implementing this over and over:

```go
type notFoundError struct {
  cause error
}

func(e notFoundError) Error() string {
  return e.cause.Error()
}

func(e notFoundError) NotFound() {}

func(e notFoundError) Cause() error {
  return e.cause
}
```

Packages can instead just do:

```
  errdefs.NotFound(err)
```

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/commit.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/docker/distribution/reference"
+	"github.com/docker/docker/api/errdefs"
 	"github.com/docker/docker/api/types/backend"
 	containertypes "github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/builder/dockerfile"
@@ -136,12 +137,12 @@ func (daemon *Daemon) Commit(name string, c *backend.ContainerCommitConfig) (str
 
 	if container.IsDead() {
 		err := fmt.Errorf("You cannot commit container %s which is Dead", container.ID)
-		return "", stateConflictError{err}
+		return "", errdefs.Conflict(err)
 	}
 
 	if container.IsRemovalInProgress() {
 		err := fmt.Errorf("You cannot commit container %s which is being removed", container.ID)
-		return "", stateConflictError{err}
+		return "", errdefs.Conflict(err)
 	}
 
 	if c.Pause && !container.IsPaused() {