try showing a stacktrace when a worker is still alive

src/managers.jl

@@ -745,13 +745,15 @@ function kill(manager::LocalManager, pid::Int, config::WorkerConfig; profile_wai
     # First, try sending `exit()` to the remote over the usual control channels
     remote_do(exit, pid)
 
+    # If the worker is still alive later, give a useful stacktrace
+    _stacktrace = stacktrace()
 
     timer_task = @async begin
         sleep(exit_timeout)
 
         # Check to see if our child exited, and if not, send an actual kill signal
         if !process_exited(config.process)
-            @warn "Failed to gracefully kill worker $(pid)"
+            @warn "Failed to gracefully kill worker $(pid)" exception=(ErrorException("Failed to gracefully kill worker $(pid)"), _stacktrace)
             profile_sig = Sys.iswindows() ? nothing : Sys.isbsd() ? ("SIGINFO", 29) : ("SIGUSR1" , 10)
             if profile_sig !== nothing
                 @info("Sending profile $(profile_sig[1]) to worker $(pid)")