Fix error parsing for valid JSON responses

[averche]

Description

When parsing error responses, if the message does not include "errors" or "error" JSON elements, the ResponseError.RawResponseBytes will be populated:

s, err := client.Secrets.KvV2Read(ctx, "my-secret", vault.WithMountPath("secret"))
if err != nil {
    var e *vault.ResponseError
    if errors.As(err, &responseError) {
        log.Printf("response status code: %d", e.StatusCode)
        log.Printf("response body: %s", e.RawResponseBytes)
    } else {
        log.Println(err)
    }
}
assert(s == nil)

Resolves #246

How has this been tested?

Added a unit test.