Serializing mode should capture stderr (optionally)

[alexellis]

To match the classic watchdog 1-for-1 the serializing mode should capture stderr (optionally)

When combine_output is set to false stderr goes to the container logs, when true it comes back in the function response.

Example for testing: port=8081 mode=serializing fprocess="stat x" ./of-watchdog

When calling localhost:8081 you should see the output of stderr either in the container logs or in the response - currently we see it in neither place.