fix: improve startup settings validation

[Nerivec]

• fix detection of invalid settings in configuration.yaml like (source: Z2M fail to start since update #27958):

serial: /dev/ttyUSB0

• fix detection of invalid yaml file
  • got lost in the shuffle of the onboarding addition (no longer triggered at the right time, hence no longer triggered at all)
• improve error messages

Important

Due to this fix, invalid configuration.yaml scenario that may have been ignored before will need to be fixed before Z2M can start again.

Supersedes #27965

[Koenkk]

Thanks!