Add more mount information to errors

Currently it is difficult to diagnose mount failures in overlay storage.
This issue is we are not sure of all of the mount options used when
attempting to mount the file system.

This PR adds missing data.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

drivers/overlay/overlay.go

@@ -1159,7 +1159,7 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 		}
 		mountData = label.FormatMountLabel(opts, options.MountLabel)
 		if len(mountData) > pageSize {
-			return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountData))
+			return "", fmt.Errorf("cannot mount layer, mount label %s too large %d", options.MountLabel, len(mountData))
 		}
 		mountFunc = func(source string, target string, mType string, flags uintptr, label string) error {
 			return mountFrom(d.home, source, target, mType, flags, label)
@@ -1169,7 +1169,7 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 	flags, data := mount.ParseOptions(mountData)
 	logrus.Debugf("overlay: mount_data=%s", mountData)
 	if err := mountFunc("overlay", mountTarget, "overlay", uintptr(flags), data); err != nil {
-		return "", fmt.Errorf("error creating overlay mount to %s: %v", mountTarget, err)
+		return "", fmt.Errorf("error creating overlay mount to %s, mount_data=%s: %v", mountTarget, mountData, err)
 	}
 
 	return mergedDir, nil