Improve errors for recursive schedule running

## What problem does this solve or what need does it fill?

If you do `world.try_run_schedule(StateTransition)` in a system that runs `OnEnter`/`OnExit`, then you get this error: `TryRunScheduleError(StateTransition)`. The error implies you did not add the `StatesPlugin`, when in fact the schedule was removed because you are currently *in* the schedule.

## What solution would you like?

Improve errors for `try_run_schedule` when failure is caused by trying to run a schedule recursively.

## What alternative(s) have you considered?

None.