Clean up CodeCorps.Emails.Transmissions.Receipt

# Problem

In #1332, and even before that, `CodeCorps.Emails.Transmissions.Receipt.build` is wrapped into a `with` statement with multiple points of failure.

The event handler code which sends this email also wrapps that call into multiple points of failure.

The border between these seems arbitrary and, more importantly, The behavior of the `Receipt` itself is inconsistent with all other emails.

We should move these points of failure into the event handler and keep the `Receipt` itself _infallible_.

Then, we should decide if the points of failure actually make sense or if it's too defensive. This is old code, so it's quite possible we made some mistakes there.