This is related to Notebook Issue 2942.

One of the changes in Notebook 5.2.0 was the removal of the @json_errors decorator via PR 2853. Instead, much of the equivalent code was moved into the APIHandler.write_error method. However, by virtue of the decorator's removal and the inheritance hierarchy, the JSONErrorsMixin.write_error method is called instead of APIHandler.write_error. JSONErrorsMixin.write_error doesn't handle the traceback functionality present in @json_errors (or APIHandler) and results in the breakage of the TestCustomDefaultKernel test.

Since APIHandler.write_error should be doing the equivalent of @json_errors, I commented out the subclassing of JSONErrorsMixin from some of the handler classes, but then encountered a different set of test errors - these relating to no reason field on HTTP errors. It turns out that one of the pieces of functionality in @json_errors not "transferred" to APIHandler was the appropriate setting of the reason code.

Since its not sufficient to fallback to using APIHandler, we should make the appropriate changes to JSONErrorsMixin.write_error in order to bridge the gap between Notebook 5.2 GA and the future release that addresses the APIHandler/reason issue.