Misleading error message when the run-image is not readable

[ekcasey]

Summary

When the lifecycle does not have the credentials needed to pull the run-image it prints a misleading error message claiming that the as-yet-to-be-created app-image has no layers:

failed to export: get run image top layer SHA: image <app-image-tag> has no layers

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Reproduction

Steps

1. Push a <run-image> to a private registry
2. pack build with the --run-image <run-image> flag pointing to the private run image.
3. Observe the misleading error message.

Current behavior

Error message looks like:

failed to export: get run image top layer SHA: image <app-image-tag> has no layers

Expected

Something useful like:

failed to export: unable to read run-image '<run-image>': unauthorized

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Context

lifecycle version

0.9.1

Warning This will likely require some (overdue) redesign of the imgutil library interface.

[natalieparellano]

Blocking this on imgutil refactor