Show `stderr` if it exists in cell outputs for failed tests

Currently, if a cell with the `%%sh` magic fails, `nbdev_test` shows only the raised error but no what was printed to `stderr`. I haven't double-checked, but I think both `stdout` and `stderr` should exist in the cell outputs.

Start by reading the `nbdev_test` entry point and how it handles failed cells run with the `%%sh` magic. Check what cell outputs contain for `stdout` and `stderr`; done when failed-test output includes available `stderr` alongside the raised error.

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