Refs #654 -- API: Status operations on orders resource (#640)

* API: Write operations on orders resource

* Add order API endpoint /extend/

src/pretix/api/exception.py

@@ -0,0 +1,16 @@
+from rest_framework.response import Response
+from rest_framework.views import exception_handler, status
+
+from pretix.base.services.locking import LockTimeoutException
+
+
+def custom_exception_handler(exc, context):
+    response = exception_handler(exc, context)
+
+    if isinstance(exc, LockTimeoutException):
+        response = Response(
+            {'detail': 'The server was too busy to process your request. Please try again.'},
+            status=status.HTTP_409_CONFLICT
+        )
+
+    return response