Test that the role fails, not that it sets blivet_output.failed

The latter would be a valid test as well, but a less important one, and
currently it does not work properly.

Do not use ignore_errors: yes on the test block, it makes all assert
pointless. Use a rescue task instead.

tests/tests_disk_errors.yml

@@ -25,11 +25,31 @@
                 disks: ['/dev/surelyidonotexist']
                 mount_point: "{{ mount_location }}"
 
-        - name: Check the error output
+        - name: UNREACH
+          fail:
+            msg: "this should be unreachable"
+
+      rescue:
+        - name: Check that we failed in the role
           assert:
-            that: "{{ blivet_output.failed }}"
-            msg: "Expected error message not found for missing disk"
-      ignore_errors: yes
+            that:
+              - ansible_failed_task.name != 'UNREACH'
+            msg: "Role has not failed when it should have"
+          vars:
+            # Ugh! needed to expand ansible_failed_task
+            storage_provider: blivet
+
+        # the following does not work properly,
+        # blivet_output.failed is false.
+        # - name: Show the error output
+        #   debug:
+        #     msg: "{{ blivet_output.failed }}"
+
+        # - name: Check the error output
+        #   assert:
+        #     that: blivet_output.failed | bool
+        #     msg: "Expected error message not found for missing disk"
+
 
     - name: Test for correct handling of safe_mode
       block: