Mk3 Chopper: missing config files causes frequency to read 0

As an instrument scientist, I don't want my Mk3 choppers to report 0 (a valid frequency) when there is no connection with the hardware. This caused confusion when setting up the Polaris T0 chopper, as a missing config file prevented the IOC talking to the chopper. The IOC log contained many messages saying there was no connection to the chopper hardware.

PV should have a communication alarm around it when such connection issues are present.

### Acceptance criteria
- [ ] If there are connection errors all readback PVs have invalid timeout alarm.