Display error messages as the come back from the transactional endpoint.
Clean up visualisation model.

o Stop polluting the node and relationship top-level key space with property keys;
  keep them nested within a propertiesMap field.
o This should avoid some horrible problems when your graph (legitimately) includes
  a property key "id".
o Further work still needed for styling to be able to distinguish between node id
  and property with key "id"