feat: Add `to_string` functions for toxencryptsave errors. #6254

Job	Run time
docker-bootstrap-node	1m 3s
docker-clusterfuzz	1m 34s
docker-esp32	9s
docker-fuzzer	10s
docker-toxcore-js	2m 3s
docker-windows-mingw (32)	6m 44s
docker-windows-mingw (64)	6m 42s
docker-bootstrap-node-websocket	17s
	18m 42s

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