Fix error msg in case no image produced

[haf-tech]

Fix #458 - Change fallback error message for non OCI images/results

In case task run does not produce an image, like git-clone, produce currently the oci logic an error message, which results in a wrong annotation chains.tekton.dev/signed: failed

This PR fix the error handling.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: haf-tech / name: Hafid Haddouti (1711617)

[tekton-robot]

Hi @haf-tech. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[priyawadhwa]

/ok-to-test

[priyawadhwa]

LGTM!

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: priyawadhwa

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[priyawadhwa]

/lgtm

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/storage/oci/oci.go	39.6%	40.7%	1.1