Advise users to check GAMS log file

[glatterf42]

Closes #511.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• ~[ ] Add, expand, or update documentation.~ Just adding to error message
• [x] Update release notes.

[codecov[bot]]

Codecov Report

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

Comparison is base (db569fc) 98.9% compared to head (e90d779) 98.9%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #513 +/- ## ===================================== Coverage 98.9% 98.9% ===================================== Files 44 44 Lines 4794 4795 +1 ===================================== + Hits 4743 4744 +1 Misses 51 51 ``` | [Files](https://app.codecov.io/gh/iiasa/ixmp/pull/513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa) | Coverage Δ | | |---|---|---| | [ixmp/model/gams.py](https://app.codecov.io/gh/iiasa/ixmp/pull/513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-aXhtcC9tb2RlbC9nYW1zLnB5) | `100.0% <100.0%> (ø)` | | | [ixmp/tests/test\_model.py](https://app.codecov.io/gh/iiasa/ixmp/pull/513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-aXhtcC90ZXN0cy90ZXN0X21vZGVsLnB5) | `100.0% <ø> (ø)` | |