stacker to throw correct and meaningful error messages such as with "make package"

[harnaircisco]

Require a meaningful error message reported to help easier troubleshooting stacker runs such as when incorrect keys are used within pkg.stacker.yaml . Pls see the "tag" key incorrectly used in place of "url" in the below yaml snippet.

Stacker run reported the error as "couldn't import base layer : bad image ref: " while it should have been something like : "does not look like a container image tag. The url field expects an image tag."

config:
  prerequisites:
    - build.stacker.yaml

${{bob_stacker_service}}:
  from:
    type: docker
    **tag**: ${{bob_c3_java17_base_image}}
  import:

preparing image slackmessageinterceptor...
copying /data/hdd/forks/apiserver/cmd/slackmessageinterceptor/start-interceptor.sh
loading
error: couldn't import base layer : bad image ref:
error: exit status 1