Improve ensure-reno

Follow I1334ff1f469061884b222dd99e72a989d72c68be and fail if
constraints_file contains a file name that does not exist.

Change-Id: I220bddabf24be5773c835635473e4b90e474eb09

roles/ensure-reno/tasks/main.yaml

@@ -6,13 +6,20 @@
 
 - name: Check to see if the constraints file exists
   stat:
-    path: "{{ constraints_file|default('missing') }}"
+    path: "{{ constraints_file }}"
     get_checksum: false
     get_mime: false
     get_md5: false
   register: stat_results
   when: constraints_file is defined
 
+- name: Fail if constraints file does not exist
+  fail:
+    msg: "Variable constraints_file is set but file does not exist."
+  when:
+    - constraints_file is defined
+    - not stat_results|skipped and not stat_results.stat.exists
+
 - name: Record file location
   set_fact:
     upper_constraints: "-c {{ constraints_file }}"