[localize] Fail build on missing translations

[stevenkuster]

Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Localize (@lit/localize)

Description

It would be very helpful to be able to have the lit-localize build command fail on missing translations. That way it can be used in CI to determine if code contains missing translations and prevent that from ending up in production code.

And idea would be to add a command line flag to the lit-localize build command so it can be set only during CI.

Alternatives and Workarounds

In theory you could read the output of the build command and see if it contains is missing, using canonical text as fallback. But that means you can't fail early and have to wait for the entire build to succeed first.