Scan for output errors in functional tests

In _stack_output() look for unexpected "output_error" messages.
Fix a missing output in the template resource tests.

Change-Id: I71d5d7e5800d7503d9e6015f637fe7fef5d867fe
diff --git a/functional/test_template_resource.py b/functional/test_template_resource.py
index 0d5734b..00e08b0 100644
--- a/functional/test_template_resource.py
+++ b/functional/test_template_resource.py
@@ -180,6 +180,9 @@
 resources:
   secret1:
     type: OS::Heat::RandomString
+outputs:
+  nested_str:
+    value: {get_attr: [secret1, value]}
 '''
         stack_identifier = self.stack_create(
             template=self.main_templ,