ReadonlyerrorThe fault as MELCloud words it, in the account's language, when the
dialect carries a text on the unit (Classic ErrorMessages); null
on a dialect that only flags it (Home) and while there is no fault.
Readonlyistrue while the unit reports a fault of its own (Classic HasError,
Home isInError).
Cross-dialect fault contract: what the unit ITSELF reports — a fault MELCloud relays (an indoor/outdoor communication error, a sensor failure), never one the cloud or this SDK raised. Orthogonal to AvailabilityAware: a faulted unit usually stays reachable and keeps answering writes, which is exactly why a consumer must say so.