prevent instance group delete if running jobs

* related to https://github.com/ansible/ansible-tower/issues/7936

awx/api/exceptions.py

@@ -0,0 +1,18 @@
+# Copyright (c) 2018 Ansible by Red Hat
+# All Rights Reserved.
+
+# Django
+from django.utils.translation import ugettext_lazy as _
+
+# Django REST Framework
+from rest_framework.exceptions import ValidationError
+
+
+class ActiveJobConflict(ValidationError):
+    status_code = 409
+
+    def __init__(self, active_jobs):
+        super(ActiveJobConflict, self).__init__({
+            "error": _("Resource is being used by running jobs."),
+            "active_jobs": active_jobs
+        })