Use ExecFuture to raise sendmail error codes out of PHPMailer

Summary:
Ref T2843. We currently drop any stdout/stderr emitted by sendmail. Instead, use `ExecFuture` so we'll throw an exception with debugging information preserved.

@tido, can you apply this and restart the daemons?

Test Plan: Rests on @tido

Reviewers: tido, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2843

Differential Revision: https://secure.phabricator.com/D5464