Trap non-zero process return values in _run_minizinc

[odow]

Closes #54

[codecov[bot]]

Codecov Report

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

Comparison is base (2968ff9) 97.20% compared to head (32961c7) 97.45%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #55 +/- ## ========================================== + Coverage 97.20% 97.45% +0.24% ========================================== Files 3 3 Lines 465 471 +6 ========================================== + Hits 452 459 +7 + Misses 13 12 -1 ```

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