Nicer error message page

[akirk]

In #240, for example, you arrive at a white page with just the error message (because we use die_with_error). Two options to fix this:

• Redirect to the Events homepage and add the ability to show a message there
• Show a full page error message with the usual header and footer.

[amieiro]

I think we should update the die_with_error method to show a template and the die_with_404 method to show an improved template.