When exporting project raise error if export is not possible

gns3server/handlers/api/controller/project_handler.py

@@ -231,14 +231,21 @@ class ProjectHandler:
         controller = Controller.instance()
         project = controller.get_project(request.match_info["project_id"])
 
-        response.content_type = 'application/gns3project'
-        response.headers['CONTENT-DISPOSITION'] = 'attachment; filename="{}.gns3project"'.format(project.name)
-        response.enable_chunked_encoding()
-        # Very important: do not send a content length otherwise QT closes the connection (curl can consume the feed)
-        response.content_length = None
-        response.start(request)
+
+        started = False
 
         for data in project.export(include_images=bool(request.GET.get("include_images", "0"))):
+            # We need to do that now because export could failed and raise an HTTP error
+            # that why response start need to be the later possible
+            if not started:
+                response.content_type = 'application/gns3project'
+                response.headers['CONTENT-DISPOSITION'] = 'attachment; filename="{}.gns3project"'.format(project.name)
+                response.enable_chunked_encoding()
+                # Very important: do not send a content length otherwise QT closes the connection (curl can consume the feed)
+                response.content_length = None
+                response.start(request)
+                started = True
+
             response.write(data)
             yield from response.drain()