lua worker.bg_worker: also report missing cqueues

Otherwise people could get confusing errors like:

attempt to index field 'bg_worker' (a nil value)

Fixes #562 (closed) (kind-of)

At least in prefill case the error unfortunately won't show that it was "caused" by the prefill module, but perhaps that's OK-ish.

daemon/lua/sandbox.lua.in

@@ -634,8 +634,9 @@ if has_cqueues then
 	end
 else
 	-- Disable asynchronous execution
-	local function disabled () error('cqueues are required for asynchronous execution') end
+	local function disabled () error('lua cqueues are required for asynchronous execution') end
 	worker.sleep = disabled
 	worker.map = disabled
 	worker.coroutine = disabled
+	worker.bg_worker = setmetatable({}, { __index = disabled })
 end