Make ValueObservation error handling mandatory

[groue]

Hiding database errors from user's sight was not a good idea.

// BEFORE: GRDB 4
do {
    try observation.start(in: dbQueue) { value in
        print("fresh value: \(value)")
    }
} catch { ... }

// NEW: GRDB 5
observation.start(
    in: dbQueue,
    onError: { error in ... },
    onChange: { value in print("fresh value: \(value)") })