Fix a misleading error message

Format conversions should have been rejected earlier; this
conceptually means something else has probably changed.

That's not _actually_ reliably true at this point because
determineListConversion does not take cannotModifyManifestListReason
into account, but that's a different bug, to be fixed some other time.

Signed-off-by: Miloslav Trmač <[email protected]>

Author: mtrmac

image/copy/multiple.go

@@ -318,7 +318,7 @@ func (c *copier) copyMultipleImages(ctx context.Context) (copiedManifest []byte,
 		// If we can't just use the original value, but we have to change it, flag an error.
 		if !bytes.Equal(attemptedManifestList, originalManifestList) {
 			if cannotModifyManifestListReason != "" {
-				return nil, fmt.Errorf("Manifest list must be converted to type %q to be written to destination, but we cannot modify it: %q", thisListType, cannotModifyManifestListReason)
+				return nil, fmt.Errorf("Manifest list was edited, but we cannot modify it: %q", cannotModifyManifestListReason)
 			}
 			logrus.Debugf("Manifest list has been updated")
 		} else {