daemon/images: rename err-returns to prevent shadowing

Prevent accidentally shadowing the error, which is used in a defer, and
while at it, also fixed some linting warnings about unhandled errors.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/images/image_commit.go

@@ -78,14 +78,14 @@ func (i *ImageService) CommitImage(ctx context.Context, c backend.CommitConfig)
 	return id, nil
 }
 
-func exportContainerRw(layerStore layer.Store, id, mountLabel string) (arch io.ReadCloser, err error) {
+func exportContainerRw(layerStore layer.Store, id, mountLabel string) (arch io.ReadCloser, retErr error) {
 	rwlayer, err := layerStore.GetRWLayer(id)
 	if err != nil {
 		return nil, err
 	}
 	defer func() {
-		if err != nil {
-			layerStore.ReleaseRWLayer(rwlayer)
+		if retErr != nil {
+			_, _ = layerStore.ReleaseRWLayer(rwlayer)
 		}
 	}()
 
@@ -93,23 +93,21 @@ func exportContainerRw(layerStore layer.Store, id, mountLabel string) (arch io.R
 	// mount the layer if needed. But the Diff() function for windows requests that
 	// the layer should be mounted when calling it. So we reserve this mount call
 	// until windows driver can implement Diff() interface correctly.
-	_, err = rwlayer.Mount(mountLabel)
-	if err != nil {
+	if _, err := rwlayer.Mount(mountLabel); err != nil {
 		return nil, err
 	}
 
 	archive, err := rwlayer.TarStream()
 	if err != nil {
-		rwlayer.Unmount()
+		_ = rwlayer.Unmount()
 		return nil, err
 	}
 	return ioutils.NewReadCloserWrapper(archive, func() error {
-			archive.Close()
-			err = rwlayer.Unmount()
-			layerStore.ReleaseRWLayer(rwlayer)
-			return err
-		}),
-		nil
+		_ = archive.Close()
+		err := rwlayer.Unmount()
+		_, _ = layerStore.ReleaseRWLayer(rwlayer)
+		return err
+	}), nil
 }
 
 // CommitBuildStep is used by the builder to create an image for each step in