Send nightly-build failures via email

Bug: misc:50

Change-Id: I679b23c1cf05594a5915856b05c160b855102d83
diff --git a/zuul.d/pipelines.yaml b/zuul.d/pipelines.yaml
index d975432..0a23d2c 100644
--- a/zuul.d/pipelines.yaml
+++ b/zuul.d/pipelines.yaml
@@ -65,3 +65,6 @@
     trigger:
       timer:
         time: 2 0 * * *
+    failure:
+      smtp:
+        subject: 'gerrit @nightly-build failed'