Fall back to global translations in Component.translate

[afurm]

What are you trying to accomplish?

Component.translate (added in #1656) blows up for any key that isn't in the component's sidecar file:

TranslatableComponent.t(".missing")   # => UncaughtThrowError: uncaught throw :exception
TranslatableComponent.t("from.rails") # => UncaughtThrowError, even though the key is in config/locales
MyComponent.t(".message")             # no sidecar file => NoMethodError: undefined method 'translate' for nil

The I18n backend throws :exception for a missing key and expects I18n.translate to catch it. The class method calls the backend directly, so nothing catches it. For components without sidecar translations, __vc_i18n_backend is nil.

What approach did you choose and why?

Do what the instance method already does: try the sidecar backend first, catch the missing translation, and fall back to the global translations, here through I18n.translate, since there's no view context on the class. A missing key now returns the usual "Translation missing" string, and default: and raise: true behave like they do with I18n.t. When there are no sidecar translations, it goes straight to I18n.translate.

Anything you want to highlight for special attention from reviewers?

The class method still doesn't do the HTML-safe handling for _html keys. #1656 left that out on purpose, so I didn't touch it.