Fix running with CasADi 3.5, and bump CasADi requirement

From CasADi 3.5.0 onwards, a solver failure would return in an Exception being thrown by CasADi. To get the old behavior (like 3.4.5) back, we have to set error_on_fail to False. This option is not recognized by 3.4.5. Instead of an if-else on CasADi version, we just bump the requirement as other things are likely to be different as well.