fs/littlefs: Log error code on failed flash_area_open

Log error code to make it easier to investigate problems.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

subsys/fs/littlefs_fs.c

@@ -657,7 +657,7 @@ static int littlefs_flash_init(struct fs_littlefs *fs, void *dev_id)
 	/* Open flash area */
 	ret = flash_area_open(area_id, fap);
 	if ((ret < 0) || (*fap == NULL)) {
-		LOG_ERR("can't open flash area %d", area_id);
+		LOG_ERR("can't open flash area %d, err %d", area_id, ret);
 		return -ENODEV;
 	}