fix: handle error from InvalidateAllCaches in slack.go (#30606)

* fix: hanlde error from InvalidateAllCaches in slack.go

* change signature of InvalidateAllCaches to *model.AppError

* return  err from InvalidateAllCaches everywhere

* Formatting

---------

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

server/channels/app/admin.go

@@ -145,8 +145,8 @@ func (s *Server) InvalidateAllCaches() *model.AppError {
 	return s.platform.InvalidateAllCaches()
 }
 
-func (s *Server) InvalidateAllCachesSkipSend() {
-	s.platform.InvalidateAllCachesSkipSend()
+func (s *Server) InvalidateAllCachesSkipSend() *model.AppError {
+	return s.platform.InvalidateAllCachesSkipSend()
 }
 
 func (a *App) RecycleDatabaseConnection(rctx request.CTX) {