fix(firestore-stripe-invoices): Change status to 404 when invoice is not found in firestore on updateInvoice

[kazuooooo]

I use stripe-invoices with stripe-payment.
Every time I create a subscription(which creates an invoice), the updateInovice hook is called by invoice.created, and raises a 500 error.

I think the status code should be 404, not 500, or invoice.created should not be included in the documentation.

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