The event-sourcing config file has a key, catch_exceptions, that determines what will happen should a projector or reactor throw an exception. If this setting is set to false, exceptions will not be caught and your app will come to a grinding halt.
If catch_exceptions is set to true, and a projector or reactor throws an exception, all other projectors and reactors will still get called. The Projectionist will catch all exceptions and fire the EventHandlerFailedHandlingEvent. That event contains these public properties:
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eventHandler: The projector or reactor that could not handle the event.
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storedEvent: The instance of Spatie\EventSourcing\Models\StoredEvent that could not be handled.
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exception: The exception thrown by the EventHandler.
It will also call the handleException method on the projector or reactor that threw the exception. It will receive the thrown error as the first argument. If you throw an exception in handleException, the Projectionist will not catch it and your php process will fail.