respect DEBUG_PROPAGATE_EXCEPTIONS

I love `DEBUG_PROPAGE_EXCEPTIONS` in my local settings when doing development. If an error occurs in an AJAX request, the only way to find out what the message was is to use the web console (if you have request logging enabled) and look at the output in JSON. 

This setting is built-in since 1.3 series I think and it's still in the [latest master](https://github.com/django/django/blob/524e71c9c20da57722b22b978d87690379ca4ade/django/core/handlers/base.py#L233).