SacessOptimizer: avoid FileNotFoundError if run with ridiculously low…

… walltime limit (#1177)

It was possible that SacessOptimizer would quit before even starting the first
iteration if a tiny walltime limit is set. This led to a FileNotFoundError when
trying to collect the temporary results. Fixed here by starting at least one
iteration.

Closes #1172

pypesto/optimize/ess/sacess.py

@@ -471,7 +471,8 @@ def run(
         i_iter = 0
         ess_results = pypesto.Result(problem=problem)
 
-        while self._keep_going():
+        # run ESS until exit criteria are met, but start at least one iteration
+        while self._keep_going() or i_iter == 0:
             # run standard ESS
             ess, cur_ess_results = self._run_ess(
                 refset=refset,