Nexus: Improve + surface QMC optimization handling

**Is your feature request related to a problem? Please describe.**

Nexus has logic to handle failed optimization runs and provide a productive experience, e.g. continuing if an optimization looks "good enough" even if not completed. After #5605 , the logic needs improving for robustness + it is worth documenting and perhaps moving the implementation of this to a higher level in Nexus.

**Describe the solution you'd like**

Briefly documented + robust implementation, say, requiring min(3? 5?, requested_number_of_cycles) optimization steps to complete and for sensible v/e ration before continuing. Should check that full cycle of config generation actually completed & optimization output files are present as part of go/no go check.

This is worth mentioning in the manual since it is a very convenient feature and also different behavior from directly run QMCPACK.

**Additional context**

Added after initial discussions Re: #5605