AO3-6790 Raise 404 when attempting to edit pseud that doesn't exist

[lou-codes]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• 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/AO3-6790

Purpose

When navigating to an edit page for a pseud that doesn't exist (/users/valid_user/pseuds/fake_pseud/edit), we should return a 404 error

Testing Instructions

Steps to reproduce, from JIRA:

User

1. Log in
2. Attempt to access the edit pseud page for a nonexistent pseud on your account. For example, if you are logged in as testy and don't have a pseud named annoying_error, enter the following URL in your browser: https://test.archiveofourown.org/users/testy/pseuds/annoying_error/edit

Admin

1. Log in as an admin who can edit pseuds (policy_and_abuse or superadmin)
2. Attempt to access the edit pseud page for a nonexistent pseud on an existing user. For example, if the user testy exists but doesn't have a pseud annoying_error, enter the following URL in your browser: https://test.archiveofourown.org/users/testy/pseuds/annoying_error/edit will give a 500 error

References

Are there other relevant issues/pull requests/mailing list discussions?
Similar to linked ticket AO3-6791 to give 404 when the user doesn't exist, PR here

Credit

lou (she/her)

[Bilka2]

Thank you!

Just a note, you don't have to copy the testing instructions from the Jira issue, it's fine to just put something like "Refer to Jira" into that section in that case.