Added handing for simpler error message for Unknown Dockerfile instructions.
Added handing for simpler error message for Unknown Dockerfile instructions.

Signed-off-by: William Henry <whenry@redhat.com>

Closes: #457
Approved by: rhatdan

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