How to make Django template raise an error if a variable is missing in context

There is a Django Snippet which provides a solution:

# settings.py
class InvalidVarException(object):
    def __mod__(self, missing):
        try:
            missing_str=unicode(missing)
        except:
            missing_str="Failed to create string representation"
        raise Exception('Unknown template variable %r %s' % (missing, missing_str))
    def __contains__(self, search):
        if search=='%s':
            return True
        return False

TEMPLATE_DEBUG=True
TEMPLATE_STRING_IF_INVALID = InvalidVarException()