Improve error message when no sigs/atts are found for an image (#2101)

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

pkg/cosign/fetch.go

@@ -75,7 +75,7 @@ func FetchSignaturesForReference(ctx context.Context, ref name.Reference, opts .
 		return nil, fmt.Errorf("fetching signatures: %w", err)
 	}
 	if len(l) == 0 {
-		return nil, fmt.Errorf("no signatures associated with %v: %w", ref, err)
+		return nil, fmt.Errorf("no signatures associated with %s", ref)
 	}
 
 	signatures := make([]SignedPayload, len(l))
@@ -126,7 +126,7 @@ func FetchAttestationsForReference(ctx context.Context, ref name.Reference, opts
 		return nil, fmt.Errorf("fetching attestations: %w", err)
 	}
 	if len(l) == 0 {
-		return nil, fmt.Errorf("no attestations associated with %v: %w", ref, err)
+		return nil, fmt.Errorf("no attestations associated with %s", ref)
 	}
 
 	attestations := make([]AttestationPayload, len(l))