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/start_unix.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 
 	"github.com/containerd/containerd/linux/runctypes"
+	"github.com/docker/docker/api/errdefs"
 	"github.com/docker/docker/container"
 	"github.com/pkg/errors"
 )
@@ -16,7 +17,7 @@ func (daemon *Daemon) getRuntimeScript(container *container.Container) (string,
 	name := container.HostConfig.Runtime
 	rt := daemon.configStore.GetRuntime(name)
 	if rt == nil {
-		return "", validationError{errors.Errorf("no such runtime '%s'", name)}
+		return "", errdefs.InvalidParameter(errors.Errorf("no such runtime '%s'", name))
 	}
 
 	if len(rt.Args) > 0 {