Cerr

`std::cerr` is only meant for errors, and possibly debugging info.

One of the most annoying thing is the use of `cerr` in bench. Some info comes out in `cout`, and some info in `cerr`, which means user has to resort to some tricky shell manipulation:

    ./stockfish bench 2>&1|grep "Nodes\/second"

in order to redirect `cerr` into `cout` so that `grep` can find the lines desired, because `cerr` does not flow into the pipe that `grep` reads from...

No functional change.