Handle scheduler shutdown in `MasterService`

[DaveCTurner]

Today if ThreadPool#scheduler is shut down while submitting a task for execution by the MasterService then we directly throw the rejection exception to the caller. By the looks of it most callers don't expect this method to throw anything, so this commit adjusts the behaviour to fail the task instead.

Closes #105549

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)