Fixes error interpolation case where err is nil

[ryanmoran]

Summary

In cases where the checksums for dependencies don't match, we were outputting an error message that looked broken:

[builder]   Executing build process
[builder]     Installing Node Engine 16.13.2
[builder] checksum does not match: %!s(<nil>)

This was caused by our attempt to interpolate nil as a string in the error message. We don't need this interpolation because we have already checked that the error is nil in prior statements.

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).

[sophiewigmore]

This test is testing

packit/postal/service.go

Lines 172 to 175 in 86f0343

	err = vacation.NewArchive(validatedReader).WithName(name).StripComponents(dependency.StripComponents).Decompress(layerPath)
	if err != nil {
	return err
	}

I think. Is that intended?

[ryanmoran]

Yeah. I missed that we had an earlier test that asserts we handle the error coming back from the decompression. Unfortunately, I've been unable to create a test case that covers what I really want to test, which is the validatedReader.Valid() conditional. I can't seem to create a tarball that triggers the situation I am able to see in a real dependency in a real buildpack.

[ForestEckhardt]

What buildpacks are you seeing this error in?

[ForestEckhardt]

I have constructed a buffer that can get to the second failure state. https://gist.github.com/ForestEckhardt/84eb2305c4626e3b93a5457baed20764