Explicit error message when tabular data url can't be retrieved (404)

validata_ui/views.py

@@ -685,12 +685,18 @@ def custom_validator():
 
         # Process URL
         else:
+            validation_form_url = compute_validation_form_url(
+                schema_instance.request_parameters()
+            )
+
             if not url_param:
                 flash_error("Vous n'avez pas indiqué d'URL à valider")
-                return redirect(
-                    compute_validation_form_url(schema_instance.request_parameters())
-                )
-            return validate(schema_instance, URLValidataResource(url_param))
+                return redirect(validation_form_url)
+            try:
+                return validate(schema_instance, URLValidataResource(url_param))
+            except frictionless.FrictionlessException as ex:
+                flash_error(ex.error.message)
+                return redirect(validation_form_url)
 
     elif request.method == "POST":