Bug 12426: Simplify the code adding a new subroutine GetMessage

 Bug 12426: Simplify the code adding a new subroutine GetMessage

The C4::Letters module does not have a GetMessage subroutine, which
could be quite useful.
This patch adds it and simplifies the code added by the previous patch.
It also adds a few tests and fixes POD typos.

Note that ResendNotice only resends failed messages. This will avoid to
resend already sent messages (using an url from the browser history for
instance).

Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>