Add Translatable Page Title to Download File Error Page

Hi there!

When a user attempts to download a file on a store that requires a user to be logged in they get an error message which is a bit abrasive.
http://cld.wthms.co/IGEc

We should change the default title to something a bit more friendly like "Log in to Download Files".
http://cld.wthms.co/CONm

A site owner should also be given the opportunity to change this text so I wrapped it in a translation wrapper function.

See [support ticket](https://woothemes.zendesk.com/agent/#/tickets/60893) for reference.