Improve error message when custom arg decode fails

[ferranbt]

No description provided.

[codspeed]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 64 skipped benchmarks¹

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_{Comparing improve-error-message-decode-custom-arg (14b0757) with main (24e626a)}

Footnotes

1. 64 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩