Detect exited threaded mapping workers

tests/utilities/test_threaded_map_regressions.py

@@ -0,0 +1,23 @@
+import subprocess
+import sys
+
+import pytest
+
+
+@pytest.mark.parametrize("exception", ["SystemExit", "KeyboardInterrupt"])
+def test_exited_thread_is_reported(exception: str) -> None:
+    script = f"""
+from great_ai.utilities import threaded_parallel_map, WorkerException
+def dead(value):
+    raise {exception}()
+try:
+    list(threaded_parallel_map(dead, [1], concurrency=1))
+except WorkerException as error:
+    assert 'exited unexpectedly' in str(error)
+else:
+    raise AssertionError('Worker exit was ignored')
+"""
+    result = subprocess.run(
+        [sys.executable, "-c", script], capture_output=True, text=True, timeout=10
+    )
+    assert result.returncode == 0, result.stderr