Gracefully handle 403, 404, 500 errors.

[aaron-schroeder]

I might end up needing to do each thing twice - once for main flask app, once for dash pages.

References

• Grinberg:
  • https://github.com/miguelgrinberg/flasky/blob/master/app/main/errors.py
  • https://github.com/miguelgrinberg/flasky/blob/bebad2ec9942bac6e3ecafb99f2e074ff8946e30/app/api/errors.py
• https://community.plotly.com/t/how-to-create-404-error-page-in-dash/42464/7
• How dash implements 404 handling