Ignore when helper files are missing

[BarDweller]

Summary

The helper buildpack is designed to run when the extension runs, but is intended to be a no-op if run by mistake. This allows it to be present in other builders that may not even contain the extension. Today, this fails, because the helper buildpack expects all errors reading the files to be build errors.

Use Cases

A user includes the buildpack manually, or runs the builder image on a platform that does not support image extensions

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[dmikusa]

Looks good to me, but you might want to add test coverage for these cases to prevent a possible regression in the future.