Error on Single PV blocks entire data query for a given date

[apcarp]

Looks like if a single PV is not found in the archiver, then the query for all of the data for all of the cavities for a given date fails. Probably better to just return null (similar to a Mya <> event), and log a warning. Right now this is happening because a CM is being upgraded to a C75, the IOC is down, the CED has been updated, and Mya doesn't have the new C75-related PVs.

[apcarp]

I'm honestly not sure what to do here. If the query fails in part or whole, should we present what data we have? Since some of the reports are aggregatations of the individual cavity data (think EGAIN of a cryomodule) and those aggregations are likely wrong I would rather not display it. In most (all?) cases, the PVs would eventually become available and the correct data would then be put in the cache for later display. I will leave the error state as is since I'd rather have a problem being reported than insipidly corrupting results.