Horizon defines classes of exception from the services it
communicates with. In the plugin case, the plugin needs to set
up including plugin content specific exceptions to the classes of
exceptions in Horizon. This patch includes this content.
Setting up an extensible exception list in exceptions.py and
including the exceptions in the configuration files.
Change-Id: Ie91cfe1e0f34a710a96b4a43cf62af35d989920c
David Lyle