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/.golangci.yml

@@ -103,7 +103,6 @@ issues:
         channels/app/platform/status.go|\
         channels/app/platform/web_hub_test.go|\
         channels/app/post_test.go|\
-        channels/app/slack.go|\
         channels/app/slashcommands/auto_environment.go|\
         channels/app/slashcommands/command_test.go|\
         channels/app/slashcommands/helper_test.go|\