Improve CLI error handling a bit

Reduce the number of *easy* ways to hit UB or an uncaught exception. This builds on Sopel's work on FEN validation.

Obviously SF is not and will never be a security boundary, but at least we can reduce the number of reports about uncaught `std::stoi` out of range, etc.

I also added a gentle fallback to `NumaConfig` specifically because that's something that (as a dev) I frequently set by hand in the CLI, but happy to revert if that's too annoying of a change.

closes https://github.com/official-stockfish/Stockfish/pull/6844

No functional change

src/position.cpp

@@ -1593,7 +1593,7 @@ bool Position::upcoming_repetition(int ply) const {
 
 // Flips position with the white and black sides reversed. This
 // is only useful for debugging e.g. for finding evaluation symmetry bugs.
-void Position::flip() {
+std::optional<PositionSetError> Position::flip() {
 
     string            f, token;
     std::stringstream ss(fen());
@@ -1622,9 +1622,7 @@ void Position::flip() {
     std::getline(ss, token);  // Half and full moves
     f += token;
 
-    set(f, is_chess960(), st);
-
-    assert(pos_is_ok());
+    return set(f, is_chess960(), st);
 }