Improve dataframe render invalid installation message when mitosheet is not installed

[aarondr77]

Imagine this scenario:

1. A user has Mito installed and creates a mitosheet default dataframe renderer
2. They save the Jupyter notebook
3. They share the .ipynb file with someone else
4. That new person does not have mitosheet installed
5. When they open the .ipynb file, the mitosheet is displayed with the invalid installation message

When the user reruns the code cells, the mitosheet gets replaced by the default pandas dataframe output so it's not a huge deal. But it would be good to provide a better message in this case. For example, sharing some information about Mito and how to install it.