Better errors for illegal PyO3 `FieldValue` values. #1691

Job	Run time
Check fuzz targets	16s
Check rustdoc WASM target	25s
Run JS linters	52s
Check lint and rustfmt	40s
Run tests	39s
WASM tests	52s
pre-publish checks	0s
Python tests and maturin build (3.9)	38s
Python tests and maturin build (3.10)	43s
Python tests and maturin build (3.11)	44s
Python tests and maturin build (3.12)	58s
Python tests and maturin build (3.13.0-rc - 3.13)	54s
All CI stages	1s
pre-publish checks	0s
publish trustfall_filetests_macros	0s
publish trustfall_core	0s
publish trustfall_derive	0s
publish python trustfall	0s
publish trustfall	0s
publish trustfall_stubgen	0s
	7m 42s

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