The nodehandler uses four different levels of logging messages:
Those are logging messages about various internal state changes which are mainly useful for the developers of nodehandler or for more in-depth analysis of what's going on. They should be of little use for the casual experimenter
Those logging messages report state changes which we deemed useful to report to the experimenter. An info message does NOT report anything abnormal. Info messages, as the name implies inform the experimenter that the experiment is progressing and should be considered POSITIVE feedback.
Warning messages report abnormal behavior but it is still possible for the nodehandler to proceed. It is left to the experimenter to decide if it makes sense to proceed or if the experiment should be stopped. Warning messages usually originate from the nodes. There are mechanisms which would allow an experiment script to react to those messages and potentially take corrective actions. Right now, those mechanisms aren't fully developed and therefore not properly documented.
Error messages report unexpected behaviors by the nodehandler or some of its sub system. The progress of the experiment is undetermined. An error message usually results from an unprocessed exception.
In short DEBUG and INFO are logging normal progress and can be ignored, WARN and ERROR report on abnormal behavior.