Make node's winston logger print stack traces

winstonConfig.js

// Extend a winston by making it expand errors when passed in as the 
// second argument (the first argument is the log level).
function expandErrors(logger) {
  var oldLogFunc = logger.log;
  logger.log = function() {
    var args = Array.prototype.slice.call(arguments, 0);
    if (args.length >= 2 && args[1] instanceof Error) {
      args[1] = args[1].stack;
    }
    return oldLogFunc.apply(this, args);
  };
  return logger;
}