Improve validation error

[floscha]

Currently, a ValueError is raised like below:

if response.status_code != 200:
    raise ValueError(response.text)

Instead, implement a dedicated ValidationError exception.
JSON errors look like the following:

{"object":"error",
 "status":400,
 "code":"validation_error",
 "message":"body failed validation: body.children[0].column_list.children should be defined, instead was `undefined`."}