Initial Marktplaats SDK scaffold

Clone the twdehands SDK into mrktplaats and retarget naming and defaults for app.marktplaats.nl while keeping the same request/response bodies and endpoint behavior.

errors.go

@@ -0,0 +1,28 @@
+package mrktplaats
+
+import (
+	"errors"
+	"fmt"
+)
+
+// APIError is returned when the API responds with a non-2xx status code.
+type APIError struct {
+	StatusCode int
+	Body       []byte
+}
+
+func (e *APIError) Error() string {
+	return fmt.Sprintf("mrktplaats: API error %d: %s", e.StatusCode, string(e.Body))
+}
+
+// IsNotFound reports whether err is an API 404 response.
+func IsNotFound(err error) bool {
+	var apiErr *APIError
+	return errors.As(err, &apiErr) && apiErr.StatusCode == 404
+}
+
+// IsUnauthorized reports whether err is an API 401 response.
+func IsUnauthorized(err error) bool {
+	var apiErr *APIError
+	return errors.As(err, &apiErr) && apiErr.StatusCode == 401
+}