CAEN: Improve behaviour on lost comms

As someone using or supporting CAENs in IBEX I want them to behave in a sensible fashion when they lose comms whilst running.

**Acceptance Criteria**
1. On comms loss (e.g. CAEN is no longer on the network) the IOC alarms but keeps running

**Notes**
1. Ensure that SANS2D is included in the next release for a new mini-inst release (if necessary create the deployed to checklist first)
1. On startup if the IOC cannot reach the CAEN it fails silently, Ticket #3644 exists to resolve this, it may be worth considering the two failure modes together
1. This has been seen on instruments where comms are lost and therefore control of the CAEN is lost (see Ticket #4942)
1. This seems to present in a console as not being able to hit return to get an EPICS prompt, and the console is unresponsive to Ctrl-X
1. Restarting the IBEX server will resolve the issue (although if the CAEN is still offline then the PVs will be missing as per #3644)
1. This may also be related to #4073