Canary build: save CWS upload/publish log

Also, fix a bug in the upload/publish command and fail when it exits
with a non-zero return code and doesn't fail with a ITEM_NOT_UPDATABLE
error.

Bug: twpowertools:20
Change-Id: I309e72b4a938be55782d0a7a18c1905ceb6cf304
diff --git a/playbooks/nightly-build/post.yaml b/playbooks/nightly-build/post.yaml
new file mode 100644
index 0000000..5cb8988
--- /dev/null
+++ b/playbooks/nightly-build/post.yaml
@@ -0,0 +1,4 @@
+- name: Get CWS upload/publish log
+  hosts: all
+  roles:
+    - fetch-cws-log