Collect & check existence of all nbs before converting

[kwinkunks]

Right now, the notebooks and extras are gathered, then the demos, so we don't know in advance how many we're converting and cannot check if any are missing. So you might wait for 20 to process before finding that the last is missing.

Could fix by building a dict of notebooks with their paths, checking all exist, then looping over to convert.

Thought: doing this with a dict would also take care of the issue of duplicate notebooks... although those should be flagged, because it might affect where the notebook ends up. E.g. if listed in curriculum and demos, should go to curriculum.

Previously agilescientific/geocomputing#1