Show error when latest version doesn't exist instead of failing silently

[minhaminha]

Product Description

When using Download Translations there's a tendency for it show a success message when you leave the app version field blank (telling it to use the latest version). It then defaults to the app's current version, which may or may not exist (user could've deleted the latest version and not created a new one). This is a quick fix to make that error known, telling the user to create a new version.

Technical Summary

This was just a bug I found while debugging other things. It only applies to Download Translations.

Feature Flag

Not a feature flag but a domain needs to be configured to use Transifex to access this feature.

Safety Assurance

Safety story

Tested locally. This is safe - it just raises an error when a version its going to try to use isn't valid.

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change