Protect against a potential timing error in the model creation logic

[lbustelo]

The problem was due to the following:

• Call to widget_manager.new_widget(...).then(function(model){...})
• The call to new_widget is what opens the comm with jupyter.widgets
• There is no handler for the target name in the kernel (R, Scala) yet, so it closes the comm
• The comm close is handled on the browser side before the WidgetModel is create and passed to the callback above
• If in my callback I want to respond to model.once("comm:close") it is too late

This PR is protecting in the callback and checking if the model's comm is still registered in the CommManager. This give me a chance to kick off the retry logic directly.

[lbustelo]

/cc @SylvainCorlay

[poplav]

Tested this and it looks to have resolved a main issue that was hindering system tests, #206. A timing issue related to channel initialization messages.

LGTM