bugfix: surface error message to the user after file upload #477

Current behavior

File uploads show a loading spinner while uploading, but there's no indication of success or failure after the upload is complete. The backend returns a response JSON with a status and message, and the upload handler in UploadFile.tsx already captures the response and error.

Expected behavior

Work with @sue-hntr to define what this should look like
User should know the status of the upload

Implementation notes:

In UploadFile.tsx, the upload handler already has a try/catch, but it only logs the results to the console.