imgtype: don't log at Fatal level
imgtype: don't log at Fatal level

Logging at Fatal calls os.Exit(), which keeps us from shutting down
storage properly, which prevents test cleanup from succeeding.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #162
Approved by: rhatdan

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