[python] translate: Throw TranslationMissing exception instead of KeyError

[juharris]

Description

Throw TranslationMissing exception instead of KeyError

Affected Dependencies

How has this been tested?

Python tests.

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

[juharris]

Those are the changes from master. I branched off of master instead of dev by mistake. I can rebase.

[juharris]

@Nolski I "rebased"