AO3-6791 Raise 404 when editing a pseud for a user that doesn't exist

[lou-codes]

Pull Request Checklist

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-6791

Purpose

When navigating to a pseud edit page for a user that doesn't exist (/users/fake_user/pseuds/pseud/edit), we should return a 404 error, instead of a 500

Testing Instructions

Replication steps from JIRA:

1. Log in as an admin who can edit pseuds (policy_and_abuse or superadmin)
2. Try to edit a pseud for a nonexistent user. For example, assuming the user no_testy does not exist, enter this URL in your browser: https://test.archiveofourown.org/users/no_testy/pseuds/no_testy/edit

References

Are there other relevant issues/pull requests/mailing list discussions?
Linked jira issue AO3-6790 to raise 404 when editing a pseud that doesn't exist, and its PR.

Credit

lou (she/her)