Extensions that create a Column subclass (using the DataGridColumnsHook) can throw exceptions inside Djblets. The DataGridColumnsHook hook and the parts of Djblets that call those methods have been sandboxed.
Now when a Column subclass from an extension throws an exception; Djblets logs the errors with enough information to find which method in the Column subclass threw the exception.

Unit tests have been written to make sure that functions from a Column subclass have been called, and when an exception is thrown it gets logged. Tests for DataGridColumnsHook test that the Column subclass when an exception is thrown while registering and unregistering a Column subclass it gets logged.
The tests fail without the sanboxing, and succeed with it.