Improves on error message on loading reporter (#374)

# Pull Request

<!--- Replace `#nnn` with your issue link for reference. -->

- Fixes #365

### Changes description

- Improves error message as users may have renamed the file

<img width="993" height="273" alt="2025-08-07_15-22"
src="https://github.com/user-attachments/assets/f2541180-27ea-439e-941d-165c7dafda67"
/>

### Alternative:

  - Remove naming pattern enforcement _(making sure it's a zip file)_
- @Polkas / @gogonzo do you have insights why this is so strict? (source
PR: #251)


https://github.com/insightsengineering/teal.reporter/blob/f776a9e217141b561b1537ca076115d2147d49ea/R/LoadReporterModule.R#L97

---------

Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: averissimo

R/LoadReporterModule.R

@@ -134,6 +134,12 @@ load_json_report <- function(reporter, zip_path, filename) {
       }
     )
   } else {
-    shiny::showNotification("Failed to load the Reporter file.", type = "error")
+    shiny::showNotification(
+      paste(
+        "Failed to load the Reporter file.",
+        "Please make sure that the filename starts with `report_`."
+      ),
+      type = "error"
+    )
   }
 }