Don't inherit from base.html in 500 error page

For server errors, the context passed to the template is empty,
so things like STATIC_URL and context processors don't work.

Fixes bug 1067206 for the folsom branch.

This has been adapted to folsom release and is a backport from master.

Change-Id: I5cd60523360d71b088226e4d0f87d88bac8a8df3
Merge "Don't inherit from base.html in 500 error page" into stable/folsom