TaskRunner: ensure errors have been logged

Python 3 buffers stderr, which means the exception traceback printed to
stderr might be held back until the process finishes. Instead, ensure that
it gets out promptly, before we process another element.

python/lsst/pipe/base/cmdLineTask.py

@@ -428,6 +428,11 @@ def __call__(self, args):
 
                 if not isinstance(e, TaskError):
                     traceback.print_exc(file=sys.stderr)
+
+        # Ensure all errors have been logged and aren't hanging around in a buffer
+        sys.stdout.flush()
+        sys.stderr.flush()
+
         task.writeMetadata(dataRef)
 
         # remove MDC so it does not show up outside of task context