scripts: rust: mention file name in error messages

Improve two error messages in the script by mentioning the doctest file
path from which the doctest was generated from.

This will allow, in case the conversion fails, to get directly the file
name triggering the issue, making the bug fixing process faster.

Signed-off-by: Guillaume Gomez <guillaume1.gomez@gmail.com>
Link: https://lore.kernel.org/r/20250228170530.950268-2-guillaume1.gomez@gmail.com


[ Reworded and removed an unneeded added parameter comma. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>