chroot: fix error handling in deferred funcs
chroot: fix error handling in deferred funcs

errors.Wrap(err) and friends will return nil if err is nil, so make
setting the error conditional.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

Closes: #1624
Approved by: TomSweeneyRedHat

TomSweeneyRedHat/buildah,nalind/buildah,nalind/buildah,projectatomic/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah