coredump: add error print

So we know if coredump exits with error, or there's no coredump.

Signed-off-by: yintao <yintao@xiaomi.com>

system/coredump/coredump.c

@@ -108,6 +108,7 @@ static int dumpfile_iterate(FAR char *path, dumpfile_cb_t cb, FAR void *arg)
       ret = mkdir(path, 0777);
       if (ret < 0)
         {
+          printf("Coredump mkdir %s fail\n", path);
           return -errno;
         }
     }