Added error code LFS_ERR_NOTEMPTY

As noted by itayzafrir, removing a non-empty directory should
error with ENOTEMPTY, not EINVAL

lfs.c

@@ -1740,7 +1740,7 @@ int lfs_remove(lfs_t *lfs, const char *path) {
         if (err) {
             return err;
         } else if (dir.d.size != sizeof(dir.d)+4) {
-            return LFS_ERR_INVAL;
+            return LFS_ERR_NOTEMPTY;
         }
     }