fix: make errors serializable

[alexander-akait]

This PR contains a:

• [x] bugfix
• [ ] new feature
• [ ] code refactor
• [x] test update
• [ ] typo fix
• [ ] metadata update

Motivation / Use-Case

part of https://github.com/webpack/webpack/issues/15980

Breaking Changes

No

Additional Info

No

[codecov[bot]]

Codecov Report

Patch coverage: 88.23% and project coverage change: +0.04 :tada:

Comparison is base (5e0308e) 93.33% compared to head (d106790) 93.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #516 +/- ## ========================================== + Coverage 93.33% 93.37% +0.04% ========================================== Files 4 3 -1 Lines 150 151 +1 Branches 33 33 ========================================== + Hits 140 141 +1 Misses 10 10 ``` | [Impacted Files](https://app.codecov.io/gh/webpack-contrib/less-loader/pull/516?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib) | Coverage Δ | | |---|---|---| | [src/utils.js](https://app.codecov.io/gh/webpack-contrib/less-loader/pull/516?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib#diff-c3JjL3V0aWxzLmpz) | `95.00% <87.50%> (-1.43%)` | :arrow_down: | | [src/index.js](https://app.codecov.io/gh/webpack-contrib/less-loader/pull/516?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib#diff-c3JjL2luZGV4Lmpz) | `90.00% <100.00%> (ø)` | |

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