File validation returns 500 for versions

[mirefly]

Describe the problem and steps to reproduce it:

https://addons-dev.allizom.org/en-US/developers/addon/webmail-ad-blocker/file/237880/validation.json

What happened?

It returns 500

What did you expect to happen?

No errors. (Or 404 when url is incorrect)

Anything else we should know?

I found it when visiting https://code.addons-dev.allizom.org/en-US/compare/7560/versions/124625...1508403/. It looks like it depends on the versions I chose. I am not sure why some versions have this problems, while others work well.

[diox]

We should return a 404, yeah. The FileValidation doesn't exist because it's an old file from 2013.

https://sentry.prod.mozaws.net/operations/olympia-dev/issues/6402408/

[diox]

Mentor: me

View to fix is in devhub/views.py: json_file_validation. Gotta add a try/except to catch File.validation.RelatedObjectDoesNotExist when accessing the file.validation. That will need a test.

[wagnerand]

If this is your first contribution, please refer to wiki.mozilla.org/Add-ons/Contribute/Code on how to get started.

[ioanarusiczki]

@mirefly https://addons-dev.allizom.org/en-US/developers/addon/webmail-ad-blocker/file/237880/validation.json returns a 404.
Verified on dev with FF72(Win10).