Refactor error handling

[alyssadai]

• Display error messages in alert UI elements
  • https://dash-bootstrap-components.opensource.faculty.ai/docs/components/toast/
  • https://dash-bootstrap-components.opensource.faculty.ai/docs/components/alert/
• When parsing of a new user-uploaded bagel raises errors, show alert (include problematic file name) and use dash.no_update to keep all previous inputs instead of clearing them
• Use input-filename component to display prompt to upload bagel when one has not been chosen
• Wrap any non-handled errors in an informative message for display to user as well ("Something went wrong while processing this file")

Other resources:

• https://community.plotly.com/t/informing-user-of-callback-exceptions/7870

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