Updated request failure (HTTP 400) with errCode.

zoo-api.yaml

@@ -158,6 +158,11 @@ components:
           schema:
             type: object
             properties:
+              errCode:
+                description: Some tag from a limited but growing set. Tags will not be removed, only added.
+                type: string
+                enum:
+                  - NO_MED_DATA
               err:
                 description: A plain text explanation of what's wrong with the request
                 type: string