fix: return err instead of nil on saveSnapshot failure

[Qian-Cheng-nju]

saveRaftState and saveSnapshots in internal/logdb/db.go have return nil where return err was intended:

if err := r.saveSnapshot(wb, ud); err != nil {
    return nil  // should be return err
}

When saveSnapshot fails (e.g. disk I/O error in listSnapshots), return nil causes the function to exit early without calling saveEntries or CommitWriteBatch, silently dropping the entire batch of raft log entries. The caller receives a nil error and assumes the write succeeded.

The same commit (0d704a2) has a third call site in importSnapshot that correctly writes return err.