Improve initial error message for unconfigured database

[Willard84]

I tried starting beegfs-mgmtd without running sudo /opt/beegfs/sbin/beegfs-mgmtd --init first, and these are the messages that I got. I think there is room for improvement

 sudo systemctl start beegfs-mgmtd
[sudo] password for wdavis: 
Job for beegfs-mgmtd.service failed because the control process exited with error code.
See "systemctl status beegfs-mgmtd.service" and "journalctl -xeu beegfs-mgmtd.service" for details.

 systemctl status beegfs-mgmtd.service
 beegfs-mgmtd.service - BeeGFS Management Server
     Loaded: loaded (/usr/lib/systemd/system/beegfs-mgmtd.service; disabled; preset: disabled)
     Active: failed (Result: exit-code) since Tue 2026-06-02 08:56:02 MDT; 3s ago
       Docs: https://doc.beegfs.io
    Process: 134779 ExecStart=/opt/beegfs/sbin/beegfs-mgmtd --log-target=journald (code=exited, status=1/FAILURE)
   Main PID: 134779 (code=exited, status=1/FAILURE)
        CPU: 5ms

Jun 02 08:56:02 r9-01.vm systemd[1]: Starting BeeGFS Management Server...
Jun 02 08:56:02 r9-01.vm beegfs-mgmtd[134779]: No accessible database file found at "/var/lib/beegfs/mgmtd.sqlite": No such file or directory (os error 2)
                                               If you want to initialize a new system, refer to --help or doc.beegfs.io.
Jun 02 08:56:02 r9-01.vm beegfs-mgmtd[134779]: No accessible database file found at "/var/lib/beegfs/mgmtd.sqlite": No such file or directory (os error 2)
Jun 02 08:56:02 r9-01.vm beegfs-mgmtd[134779]: If you want to initialize a new system, refer to --help or doc.beegfs.io.
Jun 02 08:56:02 r9-01.vm beegfs-mgmtd[134779]: Error: 1

In particular, the line Jun 02 08:56:02 r9-01.vm beegfs-mgmtd[134779]: If you want to initialize a new system, refer to --help or doc.beegfs.io. isn't as helpful as it could be. The user probably needs to run /opt/beegfs/sbin/beegfs-mgmtd --init, or at least we should tell them what binary should recieve the "--help" flag. Maybe something like

What about changing that error message to something like "If you want to initialize a new system, use /opt/beegfs/sbin/beegfs-mgmtd --init, or refer to doc.beegfs.io for more details."