Describe the bug
When a user try to access a version that don't exists, as "/processes/highway-copy/f/3/proposals/7/versions/-8"
It gives a 500 error rather than a 404

To Reproduce
Steps to reproduce the behavior:

1. Go to the a version of a proposal
2. Change the argument in the URL to a negative number or a letter
3. See error

Expected behavior

• raise a 404
  or
• redirect to the list of version with a flash error message

Screenshots