Parse whole file, return error message to users

[calpaterson]

Brief overview

AS A user uploading a troublesome csv

I WANT a better way to see errors

SO THAT I can correct them and re-upload

Additional details

Have some of the bits for this already, but not all

• 2-stage file upload (upload, and then parse)
  • This is implemented for git-tracking tables, but not other ways of uploading
• be fast enough to work on a 50mb file in one go
  • probably requires upgrading to psycopg3 for the new copy_expert function
• Show errors on a page to a user
  • this is already mostly implemented! it's just not used