policy.rs: refactor unstable .inspect_err feature for rust <=1.75

[eguzki]

Result's inspect_err method is unstable for rust <= 1.75, leading to

82.22 error[E0658]: use of unstable library feature 'result_option_inspect'
82.22    --> src/policy.rs:144:30
82.22     |
82.22 144 | ...                   .inspect_err(|e| debug!("#{} build_single_descriptor: failed to parse selector value: {}, error: {}",
82.22     |                        ^^^^^^^^^^^
82.22     |
82.22     = note: see issue #91345 <https://github.com/rust-lang/rust/issues/91345> for more information

This is a workaround to enable build rust on 1.75

[alexsnaps]

Why do we care about rust 1.75?