Fix linter complaints

[arik-so]

The linter is currently complaining about map_err where inspect_err is possible, as well as about a dependency_on_unit_never_type_fallback issue.

[tnull]

inspect_err doesn't exist on MSRV of 1.63, IIUC.

[arik-so]

Yup, changed it to suppressing the warnings instead.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.10%. Comparing base (23109b6) to head (d340ac3). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3334 +/- ## ========================================== + Coverage 89.61% 90.10% +0.48% ========================================== Files 126 126 Lines 102690 106287 +3597 Branches 102690 106287 +3597 ========================================== + Hits 92028 95772 +3744 + Misses 7923 7842 -81 + Partials 2739 2673 -66 ``` | [Flag](https://app.codecov.io/gh/lightningdevkit/rust-lightning/pull/3334/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lightningdevkit) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/lightningdevkit/rust-lightning/pull/3334/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lightningdevkit) | `90.10% <ø> (+0.48%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lightningdevkit#carryforward-flags-in-the-pull-request-comment) to find out more.

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[arik-so]

Alternatively, we could run the linter on 1.63?