Create a JavaScript program that checks if a given number is a prime number and handles cases where the input is not a valid integer gracefully, displaying an appropriate error message. The program should take input from the user and display the result.

function isPrime(num) {
  if (num === 1) {
    return false;
  } else if (num === 2) {
    return true;
  } else {
    for (var i = 2; i < num; i++) {
      if (num % i === 0) {
        return false;
      }
    }
    return true;
  }
}