feat: retain cause for upload fetch errors

[jefferyto]

What?

Ensure the cause for upload fetch errors (error.cause) is retained so that it gets surfaced/printed at the server console.

Why?

The error message for errors thrown by fetch (e.g. network connectivity issues, SSL certificate errors, etc.) is very generic (basically "fetch failed"); the actual error message/code is in error.cause. The current error handling for upload fetches discards the cause, making it very difficult to debug fetch errors.

How?

This updates FileRetrievalError to accept the error cause as a parameter (passing it as the data parameter of APIError), and updates the try/catch clauses in safeFetch and generateFileData to pass the error cause to FileRetrievalError.