Update HTTP exceptions

[adam-paterson]

We need to replace the Http\Client\Exception\HttpException in IBM\Watson\Core\Client\ExceptionHandler:handle with a more specific exception.

Watson error responses are usually in the format:

{
  "error": "General description of an error.",
  "errors": [
    {
      "message": "Description of a specific constraint violation.",
      "path": "The location of the constraint violation."
    }
  ],
  "code": 500
}