bfs_client and nameserver return 0 in some abnormal cases

1. when bfs.flag does not exist or nameserver has already been started, namserver can't start but return 0;
2. when touch a file already exists, bfs_client print fail message but return 0;
3. when du a directory not exist，bfs_client print fail message but return 0.

Reproduce the three cases described for nameserver and bfs_client: missing bfs.flag or an already-running nameserver, touching an existing file, and running du on a missing directory. Trace the command entry points and failure paths, then verify that each reported failure exits nonzero while successful operations retain their existing status.

Written by the indexing model from the issue text.