Replace Undefined with string

gistfile1.js

//original ember version
Ember.getPath = function(root, path) {
  var hasThis, isGlobal, ret;

  // Helpers that operate with 'this' within an #each
  if (path === '') {
    return root;
  }

  if (!path && 'string'===typeof root) {
    path = root;
    root = null;
  }

  path = cleanupStars(path);

  // If there is no root and path is a key name, return that
  // property from the global object.
  // E.g. getPath('Ember') -> Ember
  if (root === null && path.indexOf('.') < 0) { return get(window, path); }

  // detect complicated paths and normalize them
  path = normalizePath(path);
  hasThis  = HAS_THIS.test(path);

  if (!root || hasThis) {
    var tuple = normalizeTuple(root, path);
    root = tuple[0];
    path = tuple[1];
    tuple.length = 0;
  }

  return getPath(root, path);
};

var getPath = Ember.getPath;

// Overwrite the original and add the OMG 'feature'
// @pangratz solution
Ember.getPath = function(obj, path) {
  var value = getPath(obj, path);
  if (obj === App.objectWhichHasUndefinedProps) {
      return (Ember.none(value) ? 'OMG %@ is not defined!!'.fmt(path) : value);
  }
  return value;
};

To be clear, you just have to add the part starting from line #36 to your code...