Improve failure message for unused allowed untranslated keys test

[mitchellhenke]

🛠 Summary of changes

This PR makes a small proposed change to the error message around unused allowed untranslated keys from #10508 to give clearer instructions on the source of the failure and how to resolve it.

Old message:

     Failure/Error:
       expect(unused_allowed_untranslated_keys).to(
         be_empty,
         "unused allowed untranslated i18n keys: #{unused_allowed_untranslated_keys}",
       )

       unused allowed untranslated i18n keys: [{:key=>"titles.piv_cac_setup.upsell", :locales=>[:fr]}, {:key=>"titles.piv_cac_setup.upsell", :locales=>[:es]}]

Proposed message:

     Failure/Error:
           expect(unused_allowed_untranslated_keys).to(
             be_empty,
             <<~EOS,
               ALLOWED_UNTRANSLATED_KEYS contains unused allowed untranslated i18n keys.
               The following keys can be removed from ALLOWED_UNTRANSLATED_KEYS:
               #{unused_allowed_untranslated_keys.pretty_inspect}
             EOS
           )

       ALLOWED_UNTRANSLATED_KEYS contains unused allowed untranslated i18n keys.
       The following keys can be removed from ALLOWED_UNTRANSLATED_KEYS:
       [{:key=>"titles.piv_cac_setup.upsell", :locales=>[:fr]},
        {:key=>"titles.piv_cac_setup.upsell", :locales=>[:es]}]

[KeithNava]

🔥