aufs: log reason why aufs is not supported.

In case aufs driver is not supported because supportsAufs() said so,
it is not possible to get a real reason from the logs.

To fix, log the error returned.

Note we're not using WithError here as the error message itself is the
sole message we want to print (i.e. there's nothing to add to it).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/aufs/aufs.go

@@ -86,9 +86,9 @@ type Driver struct {
 // Init returns a new AUFS driver.
 // An error is returned if AUFS is not supported.
 func Init(root string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
-
 	// Try to load the aufs kernel module
 	if err := supportsAufs(); err != nil {
+		logger.Error(err)
 		return nil, graphdriver.ErrNotSupported
 	}