Ignore users.auth_failure.* as unused translation

Translation keys are used for OmniAuth callbacks, and defined by the
respective OmniAuth provider. It is expected that these strings are
unsued in the OSM website repo.

config/i18n-tasks.yml

@@ -123,6 +123,7 @@ ignore_unused:
   - 'geocoder.search_osm_nominatim.prefix.*'
   - 'javascripts.*'
   - 'doorkeeper.*'
+  - 'users.auth_failure.*'   # OmniAuth provider callback error messages
   - 'activerecord.attributes.*'
   - 'activerecord.models.*'
   - 'activerecord.help.*'