```suggestion cdef dict _CODE_MAP = { ```

```suggestion cdef dict _LEVEL_MAP = { ```

```suggestion _ERROR_STACK = ContextVar("error_stack") ```

`StackChecker` takes on all the eliminated code.

The handler looks up the thread local chained error stack, no need to pass a pointer to data.

`StackChecker` provides some familiar error checking/raising functionality, but in the context of our new Python chained error stack instead of GDAL's error stack.

New code does nothing in the debug or warning cases. Only if we have a class 3 error.

`noexcept` prevents Cython compiler warnings.

No more cramming the causing error message into the raised error message.

testing exact error messages looks fragile to me. You could potentially just test they are not empty and different between that instance and the one 3 lines below

A new test which asserts that the existing behavior on trying to read imagery from a busted GeoTIFF is preserved, and also that we can, by traversing to context and cause, get the root error.

Inside the `with` block `chaining_error_handler` can be appending objects to `error_stack`.

This context manager is used like ```python try: with stack_errors(): GDALOpenDatasetEx(...) except CPLE_BaseError as cplerr: raise RasterioError("oh no") from cplerr ```

And append the exception form of a GDAL error to it.

Here we get that Python list from the GDAL error context.

The code that adds this handler also passes a pointer to a Python list to the GDAL error context.