Better let white space in case the list is empty.

There is no easy way to i18n. We could have inserted an extra
template only for this case (an empty list) in the config module.
I think it would have been worse (annoying) than only insert
linebreaks (text email) or empty "ul" tag (HTML email).
If mail cannot be sent, report an error, don't send mail to sender
Merge pull request #530 from georchestra/i18n_extractorapp

extractorapp - missing i18n strings - fixes #283