[presolve] error message if fixing artificial variable

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c212e8b) 82.79% compared to head (65166cf) 82.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1087 +/- ## ========================================== - Coverage 82.79% 82.79% -0.01% ========================================== Files 89 89 Lines 7226 7229 +3 ========================================== + Hits 5983 5985 +2 - Misses 1243 1244 +1 ``` | [Files](https://app.codecov.io/gh/atoptima/Coluna.jl/pull/1087?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=atoptima) | Coverage Δ | | |---|---|---| | [src/Algorithm/presolve/interface.jl](https://app.codecov.io/gh/atoptima/Coluna.jl/pull/1087?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=atoptima#diff-c3JjL0FsZ29yaXRobS9wcmVzb2x2ZS9pbnRlcmZhY2Uuamw=) | `80.58% <75.00%> (-0.26%)` | :arrow_down: |

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