Enhance logging, error reporting.

[aduitsis]

• Added the onlog callback to the ww. Can be used by whoever is using the ww to ascertain progress, etc. In NDTjs the logger now sends messages to that callback too.
• Add onopen,onerror and onclose event handlers to NDTjs. The first two are trivial, but onclose tries to analyze the close event and produce some useful messages, especially in the case of abnormal websocket closure.