Add more descriptive error message for when model is infeasible

This should help users track down the source of the problem a bit more easily.

Author: alexdewar

src/simulation/optimisation.rs

@@ -161,6 +161,11 @@ pub fn solve_optimal(model: highs::Model) -> Result<highs::SolvedModel> {
         .map_err(|err| anyhow!("Incoherent model: {err:?}"))?;
 
     let status = solved.status();
+    ensure!(
+        status != HighsModelStatus::Infeasible,
+        "The solver has indicated that the problem is infeasible. It may be because the assets in \
+        this year cannot meet the required demand."
+    );
     ensure!(
         status == HighsModelStatus::Optimal,
         "Could not find optimal result for model: {status:?}"