emmc-config: Restart gracefully with no service

Right now if we restart emmc-config and `obmc-dump-monitor.service`
doesn't exist then it will fail the restart.

With the - prefix:
- Systemd ignores exit code 5 when
obmc-dump-monitor.service is missing and treats the step as successful.
- coredump-config.service will transition to active (exited)
successfully instead of failing.

Tested:
- Restart coredump-config
```
root@gocd5-bmc-01:~# systemctl status coredump-config.service
● coredump-config.service - Storage management daemon for an encrypted storage device
     Loaded: loaded (/usr/lib/systemd/system/coredump-config.service; enabled; preset: enabled)
     Active: active (exited) since Sun 2026-08-30 23:21:19 PDT; 1s ago
 Invocation: ee0d713384694dee9695159e735e2c4b
    Process: 611 ExecStart=/usr/bin/coredump-config.sh emmc (code=exited, status=0/SUCCESS)
    Process: 630 ExecStartPost=systemctl try-restart obmc-dump-monitorx.service (code=exited, status=5)
   Main PID: 611 (code=exited, status=0/SUCCESS)
        CPU: 179ms

Aug 30 23:21:18 gocd5-bmc-01.prod.google.com systemd[1]: Starting Storage management daemon for an encrypted storage device...
Aug 30 23:21:18 gocd5-bmc-01.prod.google.com coredump-config.sh[611]: coredump-config: Configuring coredump path to eMMC storage.
Aug 30 23:21:18 gocd5-bmc-01.prod.google.com coredump-config.sh[611]: coredump-config: eMMC is available, copying coredumps from /run/systemd/coredumps to eMMC.
Aug 30 23:21:18 gocd5-bmc-01.prod.google.com coredump-config.sh[622]: mv: can't remove '/mnt/luks-mmcblk0_fs/coredump/dumps': Is a directory
Aug 30 23:21:18 gocd5-bmc-01.prod.google.com coredump-config.sh[622]: mv: can't remove '/mnt/luks-mmcblk0_fs/coredump/faultlogs': Is a directory
Aug 30 23:21:19 gocd5-bmc-01.prod.google.com systemctl[630]: Failed to try-restart obmc-dump-monitorx.service: Unit obmc-dump-monitorx.service not found.
Aug 30 23:21:19 gocd5-bmc-01.prod.google.com systemd[1]: Finished Storage management daemon for an encrypted storage device.
```

- platform11: http://fusion2/3ff2006d-9c03-3f0c-99fd-bcefc6aaca1f
- platform11: http://fusion2/982d88b5-28f9-38ef-a3a2-d824b919c13c
- platform5: http://fusion2/a7e89bcd-86a0-33b2-8f33-2bb24ef3afb5
- platform15: http://fusion2/e56dcb19-6dcd-35a0-b5ee-30740fbf27df
- platform17: http://fusion2/d0e6af61-eb95-3a70-a6a3-d1bdab987ea2
- platform18: http://fusion2/cda6ba3a-58f5-3907-b584-04786b9fe634
- platform25: http://fusion2/714cc620-6f0a-3bfd-90dd-95356b62b54b
Google-Bug-Id: 482144905
Change-Id: Id7d555c0cc40a790ab433c7cb1ec495dd4fc35eb
Signed-off-by: Willy Tu <wltu@google.com>