Improve error message when no sigs/atts are found for an image

[imjasonh]

Signed-off-by: Jason Hall jason@chainguard.dev

Before this change, if you cosign download attestation $image and none were found:

Error: no attestations associated with ghcr.io/distroless/static: %!w(<nil>)

After this change:

Error: no attestations associated with ghcr.io/distroless/static

At this point in the code, the err is known to be nil -- if there was an error, you'd get the message fetching attestations: whatever error happened.

Release Note

NONE

Documentation

N/A