feat(referrer): improve referrer not found error message

[Piskoo]

This PR improves the error message when a referrer (artifact/evidence) can't be found during attestation operations.

Closes #2701

[migmartri]

I am not sure you are solving the error in the right place, you seem to be doing it at save.

Could you put examples of running the commands before and after?

[Piskoo]

I am not sure you are solving the error in the right place, you seem to be doing it at save.

Could you put examples of running the commands before and after?

Sorry, you are right, it comes from the endpoint used by our builtin chainloop.evidence() rego rule.

$ chainloop att add --value sbom.json
WRN API contacted in insecure mode
INF uploading sbom.json - sha256:bfbb8312c63447567e65f128ac05ddaebf562d072532b37fb412f47bfc32a421
ERR referrer not found
exit status 1

$ chainloop att add --value sbom.json
WRN API contacted in insecure mode
INF uploading sbom.json - sha256:bfbb8312c63447567e65f128ac05ddaebf562d072532b37fb412f47bfc32a421
ERR artifact or piece of evidence with digest sha256:bfbb8312c63447567e65f128ac05ddaebf562d072532b37fb412f47bfc32a421 not found
exit status 1