handle error in ProtoBufFile::load

https://github.com/baidu/braft/blob/e32b78a1f4973534aaa047dc2ead9167ec091b56/src/braft/protobuf_file.cpp#L87-L120

1. 117 行没有检查返回值
2. 如果执行到了112行， 且原因是实际文件长度比预期的短，返回了 -1。但读文件是成功的，errno 不会被更新。加载 log_meta时，如果发生了上述问题，且原来的 errno 正好是 ENOENT，后续逻辑会认为日志是空的，直接把 log 删光。

https://github.com/baidu/braft/blob/e32b78a1f4973534aaa047dc2ead9167ec091b56/src/braft/log.cpp#L1155-L1175

Read src/braft/protobuf_file.cpp around lines 87-120, then trace its use in src/braft/log.cpp around lines 1155-1175. Check how short reads, return values, and errno are propagated during log_meta loading. Done means failed reads cannot be treated as an empty log and all relevant error returns are handled consistently.

Written by the indexing model from the issue text.