[MM-61101] Fix errcheck issues in server/channels/app/channel.go (#28787)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -83,7 +83,6 @@ issues:
         channels/api4/websocket_test.go|\
         channels/app/bot_test.go|\
         channels/app/brand.go|\
-        channels/app/channel.go|\
         channels/app/channel_bookmark_test.go|\
         channels/app/channel_test.go|\
         channels/app/config_test.go|\

server/channels/app/channel.go

@@ -186,7 +186,9 @@ func (a *App) CreateChannelWithUser(c request.CTX, channel *model.Channel, userI
 		return nil, err
 	}
 
-	a.postJoinChannelMessage(c, user, channel)
+	if err = a.postJoinChannelMessage(c, user, channel); err != nil {
+		return nil, err
+	}
 
 	message := model.NewWebSocketEvent(model.WebsocketEventChannelCreated, "", "", userID, nil, "")
 	message.Add("channel_id", channel.Id)
@@ -710,15 +712,18 @@ func (a *App) UpdateChannelPrivacy(c request.CTX, oldChannel *model.Channel, use
 		return channel, err
 	}
 
-	if err := a.postChannelPrivacyMessage(c, user, channel); err != nil {
+	postErr := a.postChannelPrivacyMessage(c, user, channel)
+	if postErr != nil {
 		if channel.Type == model.ChannelTypeOpen {
 			channel.Type = model.ChannelTypePrivate
 		} else {
 			channel.Type = model.ChannelTypeOpen
 		}
 		// revert to previous channel privacy
-		a.UpdateChannel(c, channel)
+		if _, err = a.UpdateChannel(c, channel); err != nil {
-		return channel, err
+			a.Log().Error("Failed to revert channel privacy after posting an update message failed", mlog.Err(err))
+		}
+		return channel, postErr
 	}
 
 	a.Srv().Platform().InvalidateCacheForChannel(channel)