[MM-61462] Fix errcheck issues in server/channels/app/post.go (#29205)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/i18n/en.json

@@ -2520,6 +2520,10 @@
     "id": "api.post.create_webhook_post.creating.app_error",
     "translation": "Error creating post."
   },
+  {
+    "id": "api.post.deduplicate_create_post.cache_error",
+    "translation": "Failed to cache post after deduplicating a client repeating the same request."
+  },
   {
     "id": "api.post.deduplicate_create_post.failed_to_get",
     "translation": "Failed to fetch original post after deduplicating a client repeating the same request."