VES API Has inconsistent error responses which are currently throw a nil pointer

[Mentioum]

Currently any errors which come back from the VES API are unmarshalled into the following struct:

type ErrorResponse struct {
	Errors []Error `json:"errors"`
}

type Error struct {
	Status Status `json:"status,omitempty"`
	Code   Status `json:"code,omitempty"`
	Title  string `json:"title"`
	Detail string `json:"detail,omitempty"`
}

Unfortunately the VES API isn't so consistent. When Supplying the following test registration number ER19THR as documented in the Test API response document it is supposed to return a 429 Too Many Requests

What it actually returns:

{
    "message": "Too Many Requests"
}

This obviously fails to unMarshal into the ErrorResponse type and actually causes a nil pointer exception when the array is nil in func Do as shown below:

func (r *Request) Do(v interface{}) (*http.Response, error) {
	if r.e != nil {
		return nil, r.e
	}

	resp, err := r.c.httpClient.Do(r.Request)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != 200 {
		var errResp ErrorResponse
		err := json.NewDecoder(resp.Body).Decode(&errResp)
		if err != nil {
			return resp, err
		}
		return resp, errResp.Errors[0]
	}

I suggest creating a custom unmarshalling function for ErrorResponse which accounts for the VES API's inconsistencies.

Will make a PR when we get around to fixing it on our end if you don't fix it before us.