text: Title: Product format error message
  Body: ```
  Could not determine format for product ‘outputs/validation_result0.pkl’. Pass a valid extension (‘.html’, ‘.ipynb’, ‘.md’, ‘.pdf’, ‘.rst’, and ‘.tex’) or pass “nbconvert_exporter_name”. If you want this task to generate multiple products, pass a dictionary to “product”, with the path to the output notebook in the “nb” key (e.g. “output.ipynb”) and any other output paths in other keys)
  ```
  
  We need to support pkl as a format, or if supported but missing the dictionary key, we should adjust the error message to be specific and give an example (for instance, to pass a pickle product do `product: 'file.pkl'`). We should also include a link to our community slack channel.