Raise when there's a I18n translation missing

The translation method translate and its alias t have bang brothers: translate! and t!. They will raise I18n::MissingTranslationData on a missing translation instead of printing a string like translation missing: de.custom.failure.

To turn on raising globally, you need to replace the default exception handler. The attached initializer makes I18n just raise any exception (in a development or test environment).