Add page always generating a 500 error.

This is needed to debug a long-standing bug with character
encoding in the error message shown on server errors.
handle_error: Return UTF-8 encoded data.

Apparently, the request.error_response does not re-encode the body
like the error_page.default mechanism does. This resulted in ISO-8859-1
characters with UTF-8 HTML metadata for 500 errors translated to
Swedish.
test500: raise AssertionError to correctly end up in handle_error().
Refactor UTF-8 handling of error pages.
Merge pull request #1 from SUNET/add_test500_error_page

Add page always generating a 500 error.