There shouldn’t be a case where we see a `ValueError` on upload any
more. Our file handling should be robust enough to deal with whatever is
thrown at it.
This commit:
- adds test files with bad data (PNG files with their extensions changed to look
like spreadsheets)
- catches whatever exceptions are raised by trying to parse these files
- returns a helpful flash message to the user
Anything else should raise a `500`, eg if the file can’t be uploaded to S3.
