Placing the OpenStack exception class definitions into their
own module inside the openstack_dashboard project allows
more flexibility for deployers and others who might wish
to use the horizon module for non-OpenStack Django projects.
The patch also contains two tiny cleanup items in the
templatetag files.
Change-Id: I8b250804ad07027d40d554ad1e7ee0b5af63d466
30b58e2d03