Dispatcher shouldn't populate error content for internal server error

By populating the Core::Error object for an internal server error
(status 500) with content in the Dispatcher, it was short circuiting
the logic in Core::Error that looks for a puplic/500.html or similar
views template.

Resolves #86.
Merge branch 'Pr/fix_500_custom_page' of https://github.com/veryrusty/Dancer2 into veryrusty-Pr/fix_500_custom_page
Update Changes
Fix test on hooks that wasn't expecting the 500 page