Better modem fault handling

[diondokter]

When a modem fault happens, the handler just panics.

This can be done a lot better. https://docs.nordicsemi.com/bundle/ncs-latest/page/nrfxlib/nrf_modem/doc/fault_handling.html
For example, the modem can be rebooted or some user function could be called.

Lots of possibilities that are better.