Update pt-br.js (#15776)

i18n: Updated missing translations of `image.error` and `pageHeader.title`

src/locale/lang/pt-br.js

@@ -107,10 +107,10 @@ export default {
       hasCheckedFormat: '{checked}/{total} selecionados'
     },
     image: {
-      error: 'FAILED' // to be translated
+      error: 'Erro ao carregar imagem'
     },
     pageHeader: {
-      title: 'Back' // to be translated
+      title: 'Voltar'
     }
   }
 };