The ContinuousAddRemoveShard thready body catches exceptions and logs them but then exits without triggering a failure. Based on the ContinuousStepdown hook, I believe that the desired behavior would be to mark the test as failed.

If it is preferable to log the error but allow the test to succeed, the exception handler should ensure that the thread fully signals its idle state before exiting, so that resmoke does not wait on it indefinitely.