Only cleanup worker processes if they still exist

Previously this was attempting to cleanup processes that had already died, which
caused exceptions.

Wait for the processor to be cleaned up in cleanup_proc()

Author: JamesWrigley

src/sch/Sch.jl

@@ -406,7 +406,12 @@ function cleanup_proc(state, p, log_sink)
             delete!(WORKER_MONITOR_CHANS[wid], state.uid)
         end
     end
-    remote_do(_cleanup_proc, wid, state.uid, log_sink)
+
+    # If the worker process is still alive, clean it up
+    if wid in workers()
+        remotecall_wait(_cleanup_proc, wid, state.uid, log_sink)
+    end
+
     timespan_finish(ctx, :cleanup_proc, (;worker=wid), nothing)
 end