Improve error reporting (#68)

* Provide APIError and use Go's error wrapping

* Add generic request error

* Fix code formatting

error.go

@@ -1,10 +1,37 @@
 package gogpt
 
-type ErrorResponse struct {
-	Error *struct {
-		Code    *int    `json:"code,omitempty"`
-		Message string  `json:"message"`
-		Param   *string `json:"param,omitempty"`
-		Type    string  `json:"type"`
-	} `json:"error,omitempty"`
+import "fmt"
+
+// APIError provides error information returned by the OpenAI API.
+type APIError struct {
+	Code       *string `json:"code,omitempty"`
+	Message    string  `json:"message"`
+	Param      *string `json:"param,omitempty"`
+	Type       string  `json:"type"`
+	StatusCode int     `json:"-"`
+}
+
+// RequestError provides informations about generic request errors.
+type RequestError struct {
+	StatusCode int
+	Err        error
+}
+
+type ErrorResponse struct {
+	Error *APIError `json:"error,omitempty"`
+}
+
+func (e *APIError) Error() string {
+	return e.Message
+}
+
+func (e *RequestError) Error() string {
+	if e.Err != nil {
+		return e.Err.Error()
+	}
+	return fmt.Sprintf("status code %d", e.StatusCode)
+}
+
+func (e *RequestError) Unwrap() error {
+	return e.Err
 }