when xlro is run on a dataset which has no catalog, it appears to break (but doesn't)

[spikelynch]

Throws a big noisy 'uncaught promises' backtrace when it can't find the ro-crate-metadata.json

[spikelynch]

This is not what I thought it was. When you run it on a directory with no metadata or spreadsheet, it should

• traverse the file system and build the spreadsheet and json, and then
• use the metadata to build the HTML preview

I think the backtrace is being caused by an order-of-execution bug where the second of these steps is happening before the json file has been written out.