daemon: replace ErrExtractPointNotDirectory with errdefs

It was only used in a single location, and the ErrExtractPointNotDirectory was
not checked for, or used as a sentinel error.

This error was introduced in c32dde5baadc8c472666ef9d5cead13ab6de28ea. It was
never used as a sentinel error, but from that commit, it looks like it was added
as a package variable to mirror already existing errors defined at the package
level.

This patch removes the exported variable, and replaces the error with an
errdefs.InvalidParameter(), so that the API also returns the correct (400)
status code.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/archive.go

@@ -16,11 +16,6 @@ import (
 	"github.com/pkg/errors"
 )
 
-// ErrExtractPointNotDirectory is used to convey that the operation to extract
-// a tar archive to a directory in a container has failed because the specified
-// path does not refer to a directory.
-var ErrExtractPointNotDirectory = errors.New("extraction point is not a directory")
-
 // ContainerCopy performs a deprecated operation of archiving the resource at
 // the specified path in the container identified by the given name.
 func (daemon *Daemon) ContainerCopy(name string, res string) (io.ReadCloser, error) {
@@ -97,7 +92,7 @@ func (daemon *Daemon) ContainerArchivePath(name string, path string) (content io
 // ContainerExtractToDir extracts the given archive to the specified location
 // in the filesystem of the container identified by the given name. The given
 // path must be of a directory in the container. If it is not, the error will
-// be ErrExtractPointNotDirectory. If noOverwriteDirNonDir is true then it will
+// be an errdefs.InvalidParameter. If noOverwriteDirNonDir is true then it will
 // be an error if unpacking the given content would cause an existing directory
 // to be replaced with a non-directory and vice versa.
 func (daemon *Daemon) ContainerExtractToDir(name, path string, copyUIDGID, noOverwriteDirNonDir bool, content io.Reader) error {
@@ -239,7 +234,7 @@ func (daemon *Daemon) containerArchivePath(container *container.Container, path
 
 // containerExtractToDir extracts the given tar archive to the specified location in the
 // filesystem of this container. The given path must be of a directory in the
-// container. If it is not, the error will be ErrExtractPointNotDirectory. If
+// container. If it is not, the error will be an errdefs.InvalidParameter. If
 // noOverwriteDirNonDir is true then it will be an error if unpacking the
 // given content would cause an existing directory to be replaced with a non-
 // directory and vice versa.
@@ -288,7 +283,7 @@ func (daemon *Daemon) containerExtractToDir(container *container.Container, path
 	}
 
 	if !stat.IsDir() {
-		return ErrExtractPointNotDirectory
+		return errdefs.InvalidParameter(errors.New("extraction point is not a directory"))
 	}
 
 	// Need to check if the path is in a volume. If it is, it cannot be in a