Remove shadow BaseException class

The BaseException class defined in exceptions.py is shadowing the
built-in BaseException class of the Python exception hierarchy, which
could potentially cause confusion.

This removes the BaseException definition and replaces it with the
existing WatcherException object.  Instantiations of the
IllegalArgumentException are also changed to use the message kwarg.

Change-Id: I20abf135805c7a354924de8a5194b59fc040460a
Closes-Bug: #1535504

watcher/decision_engine/model/model_root.py

@@ -34,12 +34,12 @@ class ModelRoot(object):
     def assert_hypervisor(self, obj):
         if not isinstance(obj, hypervisor.Hypervisor):
             raise exception.IllegalArgumentException(
-                _("'obj' argument type is not valid"))
+                message=_("'obj' argument type is not valid"))
 
     def assert_vm(self, obj):
         if not isinstance(obj, vm.VM):
             raise exception.IllegalArgumentException(
-                _("'obj' argument type is not valid"))
+                message=_("'obj' argument type is not valid"))
 
     def add_hypervisor(self, hypervisor):
         self.assert_hypervisor(hypervisor)