oci-image-tool: Cleanup partially-unpacked directories on failures

[wking]

We might want a switch to turn this off, but I think the default should be to remove the unpack directory if we fail partway through the unpack.

Spun off from here.

[philips]

This issue was moved to opencontainers/image-tools#21

[wking]

On Tue, Sep 20, 2016 at 07:48:16PM -0700, Brandon Philips wrote:

This issue was moved to opencontainers/image-tools#21

PR is opencontainers/image-tools#7.