[Trivial Fix] Add document layer to error message output

The document layer that is invalid should be included in
the exception called InvalidDocumentLayer for obvious reasons.

Change-Id: Ie7fcecc96bc7667530959af34ec146b4e4a47303

deckhand/engine/layering.py

@@ -303,6 +303,7 @@ class DocumentLayering(object):
                               document.schema, document.name,
                               self._layering_policy.layer_order)
                     raise errors.InvalidDocumentLayer(
+                        document_layer=document.layer,
                         document_schema=document.schema,
                         document_name=document.name,
                         layer_order=', '.join(

deckhand/errors.py

@@ -187,9 +187,10 @@ class InvalidDocumentLayer(DeckhandException):
     * Check that the document layer is contained in the layerOrder in the
       registered LayeringPolicy in the system.
     """
-    msg_fmt = ("Invalid layer for document [%(document_schema)s] "
-               "%(document_name)s was not found in layerOrder %(layer_order)s "
-               "for provided LayeringPolicy %(layering_policy_name)s.")
+    msg_fmt = ("Invalid layer '%(document_layer)s' for document "
+               "[%(document_schema)s] %(document_name)s was not found in "
+               "layerOrder: %(layer_order)s for provided LayeringPolicy: "
+               "%(layering_policy_name)s.")
     code = 400