ONOS-2091 Installing app when already installed will now raise an error

Change-Id: I4dacd63bf4a99244b23b932d35dd9cbd088548c1
diff --git a/web/api/src/main/webapp/WEB-INF/web.xml b/web/api/src/main/webapp/WEB-INF/web.xml
index d8759fe..4a9e5ab 100644
--- a/web/api/src/main/webapp/WEB-INF/web.xml
+++ b/web/api/src/main/webapp/WEB-INF/web.xml
@@ -62,6 +62,7 @@
                 org.onosproject.rest.exceptions.BadRequestMapper,
                 org.onosproject.rest.exceptions.WebApplicationExceptionMapper,
                 org.onosproject.rest.exceptions.IllegalArgumentExceptionMapper,
+                org.onosproject.rest.exceptions.IllegalStateExceptionMapper,
                 org.onosproject.rest.resources.JsonBodyWriter,
 
                 org.onosproject.rest.resources.ApplicationsWebResource,