Gracefully die on OpenBSD if quest path not found.

Hi --

Many OpenBSD users have pointed out to me that if solarus is run and the quest path is not found, solarus does not die gracefully and instead signals an Abort Trap.

Since we know what went wrong (no quest was found), it seems unnecessary to die ungracefully. Included patch outputs a one-liner on how to properly run solarus, then calls std::exit.

This fixes the Abort Trap on OpenBSD.