fix: Show OAuth onboarding errors

[shepilov]

Summary

• surface OAuth callback and registration failures on the onboarding OAuth page
• stop the onboarding flow from continuing to sync configuration when OAuth registration fails
• add localized copy for the OAuth failure message across the shipped locale files

[taratatach]

We need to add the new key to every locale (especially es). We can use the English value since we don't have any translator.