add error code documentation

Author: mfeurer

tests/test_tasks/test_task.py

@@ -62,6 +62,9 @@ def test_upload_task(self):
                 # success
                 break
             except OpenMLServerException as e:
+                # Error code for 'task already exists'
+                # Should be 533 according to the docs
+                # (# https://www.openml.org/api_docs#!/task/post_task)
                 if e.code == 614:
                     continue
                 else: