Don't bounce mail messages if any recipient was reserved

Summary:
Ref T13222. If we receive a message and nothing processes it, we normally try to send the user an error message like "hey, nothing handled this, maybe you got the address wrong".

Just skip the "send them an error message" part if any recipient was reserved, so if you "Reply All" to a message that is "From: noreply@phabricator" you don't get a relatively unhelpful error.

This also makes sure that the "void" address doesn't generate bounces if the "From" is a valid user email address (e.g., with metamta.can-send-as-user). That is:

• Phabricator needs to send a mail with only "CC" users.
• Phabricator puts the "void" address in "To" as a placeholder.
• The "void" address happens to route back to Phabricator.

We don't want that mail to bounce to anywhere. Normally, it won't:

• From is usually "noreply@phabricator", which isn't a user, so we won't send anything back: we only send mail to verified user email addresses.
• The message will have "X-Phabricator-Sent-This-Message: true" so we won't process it at all.

...but this is another layer of certainty.

Test Plan: Used bin/mail receive-test to receive mail to an invalid, unreserved address (bounce/error email) and an invalid, reserved address (no bounce/error email).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19987