Fixed missed translatable string inside exception

A string inside an exception was not being passed to
i18n translation so it will only show up as English if it
was ever thrown.

Here is a grep I used to make sure there were no more
exception strings that were not being passed to translations
before they were thown.
grep -r 'exception' . | grep '("' | grep -v '_'

Change-Id: I51a0d6d5b1d4053c380c8be5a0e6ac4e61985b81

keystone/trust/controllers.py

@@ -113,7 +113,7 @@ class TrustV3(controller.V3Controller):
                     trust_roles.append({'id':
                                         all_role_names[rolename]['id']})
                 else:
-                    raise exception.RoleNotFound("role %s is not defined" %
+                    raise exception.RoleNotFound(_("role %s is not defined") %
                                                  rolename)
             else:
                 raise exception.ValidationError(attribute='id or name',