Make Fn::Join give a better error message

When something goes wrong at the moment users are left staring hard
at their template trying to figure out whats wrong. We can make this a
lot easier by showing them the failed object.

Change-Id: If87447b7e160591505a68d857e8ef1751aac3723

heat/engine/cfn/functions.py

@@ -302,7 +302,8 @@ class Join(function.Function):
             if s is None:
                 return ''
             if not isinstance(s, basestring):
-                raise TypeError(_('Items to join must be strings'))
+                raise TypeError(
+                    _('Items to join must be strings %s') % (repr(s)[:200]))
             return s
 
         return delim.join(ensure_string(s) for s in strings)

heat/tests/test_parser.py

@@ -852,6 +852,14 @@ class ResolveDataTest(HeatTestCase):
                                    [' ', ['foo', 'bar']]}, 'baz']]}
         self.assertEqual('foo bar\nbaz', self.resolve(raw))
 
+    def test_join_not_string(self):
+        snippet = {'Fn::Join': ['\n', [{'Fn::Join':
+                                        [' ', ['foo', 45]]}, 'baz']]}
+        error = self.assertRaises(TypeError,
+                                  self.resolve,
+                                  snippet)
+        self.assertIn('45', six.text_type(error))
+
     def test_base64_replace(self):
         raw = {'Fn::Base64': {'Fn::Replace': [
             {'foo': 'bar'}, 'Meet at the foo']}}