This commit adds proper error messages when mail sending via the
mail queue has failed. It also refactors the MailTemplateService that it
does not use duplicate code to send mails.

If subject is a string that begins with LLL:, it will be handled
as a locallang reference and translated accordingly

The only arguments to be sent should be the required ones:
selectedExtension and selectedTemplate to prevent the redirect url
from becoming to long for some web servers to handle