Add new errorCode 4 of NarrowcastProgressResponse (#522)

A new errorCode has been added to NarrowcastProgressResponse.
New number is 4. 4 means an audience of less than 50 recipients is
included as a condition of sending.

Previously, it was not possible to determine whether an error occurred
due to a lack of enough recipients or because an audience of less than
50 recipients is included as a condition of sending. This will be fixed
with the update to the Messaging API, so the description of errorCode on
OpenAPI will also be revised.

Co-authored-by: github-actions <github-actions@github.com>

Author: github-actions[bot]

linebot/messaging_api/model_narrowcast_progress_response.go

@@ -54,7 +54,7 @@ type NarrowcastProgressResponse struct {
 	FailedDescription string `json:"failedDescription,omitempty"`
 
 	/**
-	 * Error summary. This is only included with a phase property value of failed. One of:  `1`: An internal error occurred. `2`: An error occurred because there weren't enough recipients. `3`: A conflict error of requests occurs because a request that has already been accepted is retried.
+	 * Error summary. This is only included with a phase property value of failed. One of:  `1`: An internal error occurred. `2`: An error occurred because there weren't enough recipients. `3`: A conflict error of requests occurs because a request that has already been accepted is retried. `4`: An audience of less than 50 recipients is included as a condition of sending.
 	 */
 	ErrorCode int64 `json:"errorCode"`