Make frontend api calls fail gracefully.

[aviraljain99]

For example, within lib/api/files.ts, getSignedUploadURLs and getUserFiles are async methods that make network calls such as fetch and getDocs. These operations could fail and these errors (example HTTP errors) would need to be handled properly.

Similarly, where methods from lib/api/files.ts are used, such as within components/datasets/DatasetViewer.tsx (loading the user's datasets) would depend on this error handling being done correctly.