Bug 5347: Follow up - Fix warnings, check/uncheck all and makes error message transla...

Bug 5347: Follow up - Fix warnings, check/uncheck all and makes error message translatable

This patch fixes some minor problems found in late order management:

 1) Silences 2 warns in Letters.p
After patch is applied no more warns should show up in the logs.

 2) Fixes check/uncheck al
When limiting on one vendor the checkbox in the last header column
was doing nothing. I changed the checkbox to 2 links 'check all' and
'uncheck all' as it's done in other templates.

 3) Email has been sent
The message was hardcoded into the lateorders.pl file and not
translatable.
I moved it to the template and changed the wording slightly.

Note: The error message 'The bookseller has no email' comes from
Letters.pm. I didn't change that, because I was not sure where it is
used. The error message as is can not be translated and should be
moved into the templates too.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>