chore(messaging): make waitUntil warning clearer

[pkarl]

• add function name to missing waitUntil warning to improve findability when warning surfaces in cloudflare/miniflare

Suggesting this change b/c neither I nor the cloudflare devs knew what was causing this warning/error.

Currently, this warning is surfaced like this when running wrangler dev:

...and like this in cloudflare logpush (irrelevant information omitted):

{
    $cloudflare:
        {
            event: { Cron: "0 * * * *", ScheduledTimeMs: ... },
            requestId: "...",
            scriptName: "...",
            outcome: "ok",
            eventType: "scheduled",
        },
    message: "Missing `waitUntil` option.",
    $baselime:
        {
            namespace: "0 * * * *",
            requestId: "...",
            account: "...",
            service: "...",
            dataset: "cloudflare-workers",
            baselimeId: "...",
            type: "cf-worker-req",
            messagePattern: "Missing `waitUntil` option.",
        },
}