Script error control

You can place the `swatches.getByName()` call in a try block and then check to see if there was an error. I would suggest doing the same thing for the call to `layers.getByName()`. In the code below, if the layer isn't found, the script will create it. You can do something similar for the spot color if you want.

Let me know if this works for you? Cheers!

// grab reference to the current document
var doc = app.activeDocument;

// deselect all items
doc.selection = null;

// grab reference to the work layer
var layerName = "Dimensions";
var layer;
try {
  layer = doc.layers.getByName(layerName);
} catch (e) {
  alert("'" + layerName + "' layer not found so it will be created now.");
  layer = doc.layers.add();
  layer.name = layerName;
}

// grab reference to the "Dimension" color
var colorName = "Dimension";
var color;
try {
  color = doc.swatches.getByName(colorName).color;
} catch (e) {
  alert("'" + colorName + "' swatch not found!");
  // I would typically create the spot color here
  color = null;
}

// check to see if the color was found, if so move any matching items
if (color != null) {
  // find items stroked with "Dimension" color and move to layer
  doc.defaultStrokeColor = color;
  app.executeMenuCommand("Find Stroke Color menu item");
  moveSelectedItemsToLayer(layer, ElementPlacement.PLACEATEND);

  // find items filled with "Dimension" color and move to layer
  doc.defaultFillColor = color;
  app.executeMenuCommand("Find Fill Color menu item");
  moveSelectedItemsToLayer(layer, ElementPlacement.PLACEATEND);
}

// Move any selected item to `layer` at `insertLocation`.
function moveSelectedItemsToLayer(layer, insertLocation) {
  var _selectedItems = app.selection;
  if (_selectedItems.length == 0) return; // no need to continue
  for (var i = _selectedItems.length - 1; i >= 0; i--) {
    _selectedItems[i].move(layer, insertLocation);
    _selectedItems[i].selected = false;
  }
  app.redraw();
  doc.selection = null;
}