Add new errorCode 4 of NarrowcastProgressResponse (#79)

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.

Author: eucyt

messaging-api.yml

@@ -2728,6 +2728,7 @@ components:
             `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.
         acceptedTime:
           type: string
           format: date-time