core/error: add status code for before publish rule rejection

For convenience, we add this error code so that it can be easily checked
against.

[CHA-701]

src/core/errors.ts

@@ -4,6 +4,14 @@ import * as Ably from 'ably';
  * Error codes for the Chat SDK.
  */
 export enum ErrorCodes {
+  /**
+   * The message was rejected before publishing by a rule on the chat room.
+   *
+   * For example, a message may be rejected because it violates a moderation policy
+   * that bans profanity.
+   */
+  MessageRejectedByBeforePublishRule = 42201,
+
   /**
    * The messages feature failed to attach.
    */