`UnknownRequiredFeature` implies LDK requires, not the thing reading requires

[TheBlueMatt]

User pointed out they were confused by the terminology "unknown required feature", which meant the thing we were trying to read had a required feature LDK didn't understand, rather than something LDK required that wasn't there. We may even use the error for both things, which we should fix, but more docs and maybe a better name would be good.