* Print key in map.getitem/delitem KeyError

* Add tests

* Fix tests

* Make robust

* Make clang-tidy happy

* Return a Python str

* Show beginning and end of the message

* Avoid implicit conversion

* Split out `format_message_key_error_key_object()` to reduce amount of templated code.

* Use `"✄✄✄"` instead of `"..."`

Also rename variable to `cut_length`, to not get into even/odd issues with the meaning of "half".

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Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>