Add validation for notebook existence

[marthacryan]

Fixes #815. Displays this error when the notebook corresponding to a node does not exist.

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[ajbozarth]

one quick comment about scalability, I haven't had time to check this out and test it yet though

[ajbozarth]

A couple comments, one on how this related to #861 and another on scalability of the error string. Otherwise the code looks good.

I've also check it out and ran it locally and it works good. Tested it on open, run, export, and when updating properties after moving the notebook to another dir. I also checked that it works right if the notebook is moved back.

[ajbozarth]

Quick parallel dev note here:

If #861 is merged first then this will need to be updated from node.app_data.filename to PipelineService.getWorkspaceRelativeNodePath(this.widgetContext.path, node.app_data.filename);

If this is merged first then it will need to be added to #861 prior to it's merge

[karlaspuldaro]

LGTM

[ajbozarth]

LGTM, if this gets merged if you could make sure to make a review comment on #861 to update it