return HTTP status code 429 in case of rate limiting

Author: Bernhard B

src/api/api.go

@@ -414,7 +414,7 @@ func (a *Api) SendV2(c *gin.Context) {
 		case *client.RateLimitErrorType:
 			if rateLimitError, ok := err.(*client.RateLimitErrorType); ok {
 				extendedError := errors.New(err.Error() + ". Use the attached challenge tokens to lift the rate limit restrictions via the '/v1/accounts/{number}/rate-limit-challenge' endpoint.")
-				c.JSON(400, SendMessageError{Msg: extendedError.Error(), ChallengeTokens: rateLimitError.ChallengeTokens})
+				c.JSON(429, SendMessageError{Msg: extendedError.Error(), ChallengeTokens: rateLimitError.ChallengeTokens})
 				return
 			} else {
 				c.JSON(400, Error{Msg: err.Error()})