Echo back to the user the error messages that the backend responds with

[dnaaun]

The JSON API responds iwth helpful error messages such as

{
    "message": "The table needs to have the headers 'Example', and 'Category'."
}

But the frontend doesn't echo such messages back to the user.

[dnaaun]

@monajalal , can you work on this? It is all about Ajax, but that is frontend after all ..