Get exception from process (not via wait())

[raffis]

Is your feature request related to a problem? Please describe

I'd like to request a done/failed job from the scheduler and get the exception if there is one.

Describe the solution you'd like

Add a getException() to TaskScheduler\Process. Will be null if no exception and an instance of \Exception if there was one.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.