Currently, the error message is fairly opaque to a non-developer ("Cannot lock storage" or something). Instead, we should have some improvments:
- when we create the in_use.lock file, we should write the hostname/PID that locked the file
- if the lock fails, and in_use.lock exists, the error message should say something like "It appears that another namenode (pid 23423 on host foo.example.com) has already locked the storage directory."
- if the lock fails, and no lock file exists, the error message should say something like "if this storage directory is mounted via NFS, ensure that the appropriate nfs lock services are running."