Add tests for error messages and improve reader IO error position

[juntyr]

* [ ] I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.25% :tada:

Comparison is base (cc02072) 98.14% compared to head (bdcb99c) 99.40%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the GitHub App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #497 +/- ## ========================================== + Coverage 98.14% 99.40% +1.25% ========================================== Files 78 78 Lines 10308 10542 +234 ========================================== + Hits 10117 10479 +362 + Misses 191 63 -128 ``` | [Files Changed](https://app.codecov.io/gh/ron-rs/ron/pull/497?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/error.rs](https://app.codecov.io/gh/ron-rs/ron/pull/497?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2Vycm9yLnJz) | `100.00% <100.00%> (+46.37%)` | :arrow_up: | | [src/options.rs](https://app.codecov.io/gh/ron-rs/ron/pull/497?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL29wdGlvbnMucnM=) | `100.00% <100.00%> (ø)` | | | [tests/options.rs](https://app.codecov.io/gh/ron-rs/ron/pull/497?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-dGVzdHMvb3B0aW9ucy5ycw==) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.