Panic-less version of World::run_schedule

[ItsDoot]

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

As of #7911, Bevy no longer panics when a system is added to a schedule that doesn't exist yet. However, that's only for adding systems, not running them.

What solution would you like?

A version of World::run_schedule that returns a Result rather than panic, likely called World::try_run_schedule.

What alternative(s) have you considered?

Check the Schedules resource for an existing schedule before running it.

Additional Context

Ran into this issue during #7936.

[alice-i-cecile]

Remember to check and replace internal usages too after adding this method :)

[cart]

Relevant: #8079 (comment)

If we're going to embrace this pattern we really need to remove the warnings here. I'm removing them in that PR.