Avoid treating supervisord's SIGTERM as an error in CommandEventHandler

lib/tfw/components/pipe_io_event_handler.py

@@ -127,6 +127,9 @@ class CommandEventHandler(PipeIOEventHandler):
     @terminate_process_on_failure
     def _monitor_proc(self):
         return_code = self._proc.wait()
+        if return_code == -int(SIGTERM):
+            # supervisord asked the program to terminate, this is fine
+            return
         if return_code != 0:
             _, stderr = self._proc.communicate()
             raise RuntimeError(f'Subprocess failed ({return_code})! Stderr:\n{stderr.decode()}')