Bug 28803: Add unit tests

Bug 28803: Add unit tests

This patch updates Letters.t to confirm that invalid email addresses in
the message_queue should not throw an exception when sending mail but
instead set the status to failed and pass error details to the end user.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>