ggml : don't crash when backend search path can't be read (#28271)
Use std::error_code overloads of fs::current_path() and
fs::directory_iterator in ggml_backend_load_best() so an
inaccessible search path (WebDAV mount, removed CWD) is
skipped instead of terminating the process with an uncaught
filesystem_error.
Signed-off-by: Adrien Gallouët <angt@huggingface.co>