Better error reporting for bad trait definitions.

In the case someone does
   destination_trait: source_trait

instead of
   destination_trait:
	fields: source_trait

it will now get a proper error message.

Change-Id: Ia22f58b6443714c2a407c7a3f94ed24f437efc1c

stackdistiller/distiller.py

@@ -105,7 +105,13 @@ class TraitDefinition(object):
         self.cfg = trait_cfg
         self.name = name
 
+        type_name = None
+        try:
             type_name = trait_cfg.get('type', 'text')
+        except AttributeError as e:
+            raise EventDefinitionException(
+                    "Unable to get type for '%s'" % trait_cfg,
+                    self.cfg)
 
         if 'plugin' in trait_cfg:
             plugin_cfg = trait_cfg['plugin']

tests/test_distiller.py

@@ -143,6 +143,13 @@ class TestTraitDefinition(DistillerTestBase):
         self.fake_plugin_map = dict(test=self.test_plugin_class,
                                     nothing=self.nothing_plugin_class)
 
+    def test_bad_field_definition(self):
+        self.assertRaises(distiller.EventDefinitionException,
+                          distiller.TraitDefinition,
+                          'test_trait',
+                          "foo",
+                          self.fake_plugin_map)
+
     def test_to_trait_with_plugin(self):
         cfg = dict(type='text',
                    fields=['payload.instance_id', 'payload.instance_uuid'],