Global permission Allow anonymous requests doesn't work properly

Firslty, if the user isn't logged at JIRA and with the global setting Anonymous requests enabled he gets an error 500 while he should be able to export the current information with the template pre-installed. The error printed on log is attached to the current issue.

Secondly, when trying to export an issue through an anonymous request, with the "Anonymous requests" permission turned disabled, we should print an error 'Forbiden' instead of the error 500.