Catch Dropzone errors from XLSX

[PerryRylance]

If you drop a "CSV" file with a bunch of HTML dumped in it, the XLSX library will go looking for a table element. If no such element is present, XLSX.read throws, leaving the importer in a loading state.

We've forked this to wrap that call in a try / catch and pass this back up to the UploadStep to toast on error.