Provenance: uploading a prov file that does not conform to format is allowed to continue, throws exception on preview.

[kcondon]

1. Upload an actual example prov file that doesn't seem to conform to supported format, see warning/error on format in UI
2. Click continue, see Providence preview pop up
3. Click preview, see error page, server log error.
   server_log.txt
   testprov.json.txt

Note: the prov file came from the WC3 spec site, was an example copied from the page but may be incomplete.
A working file is here if needed for testing:
file-provenance.json.txt
Note2: .txt was added to the json files because github won't allow uploading .json so remove that when using.