optimization problem no warning when exitstatus is feasible and not optimal

[Wout4]

bug found while fuzztesting on optimization problems

An optimization problem can return a nonoptimal solution when setting a time limit.
The exit status will be feasible, and not optimal but the user is not warned about this.

Proposed fix:
for optimization problems either

• only return a solution if exit status is optimal
• warn the user when exitstatus is feasible, so the solution is not guaranteed to be optimal.

[Wout4]

this is expected behaviour!