improve trusted publishing error message

Author: marcoieni

src/controllers/krate/publish.rs

@@ -164,7 +164,7 @@ pub async fn publish(app: AppState, req: Parts, body: Body) -> AppResult<Json<Go
 
     let auth = if let Some(trustpub_token) = trustpub_token {
         let Some(existing_crate) = &existing_crate else {
-            let error = forbidden("Trusted Publishing tokens do not support creating new crates");
+            let error = forbidden("Trusted Publishing tokens do not support creating new crates. Publish the crate manually, first");
             return Err(error);
         };