nethsm: cli option check after lengthy backup verification

[ansiwen]

With nitropy nethsm restore -p backupbackup my_30MB.backup it takes a long time until this error comes:

Critical error:
Missing NetHSM host: set the --host option or the NETHSM_HOST environment variable

[robin-nitrokey]

Speeding up the backup validation (Nitrokey/nethsm-sdk-py#151) should make this much less problematic but we could still refactor the code to first check the required connection options.