Narrow exception handling for this middleware in case of django.core.urlresolvers.reverse calls.

This should fix the case when due to URI with path which is not handled by any pattern included by hostedbento.urls a 404 is raised which gets handled by 404 handler which makes use of Site.objects.get_current() which used the previous SITE_ID.
Merge pull request #19 from pbs/LUN-1042

LUN-1042 Narrow reverse() call exception handling