fix: print full error to system logs on blob upload failures

[pieh]

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Summary

Ref: https://netlify.slack.com/archives/C03ETTLQ9BP/p1765962467406159

Fixes #<replace_with_issue_number>

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[pieh]

This is somewhat simulating fetch error that often will have cause - for example like so:

TypeError: fetch failed at Object.fetch (node:internal/deps/undici/undici:11118:11) at process.processTicksAndRejections (node:internal/process/task_queues:95:5) { cause: Error: connect ECONNREFUSED 127.0.0.1:1337 at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1300:16) { errno: -111, code: 'ECONNREFUSED', syscall: 'connect', address: '127.0.0.1', port: 1337 } } 

As fetch failed is just category of error that can have multiple underlying causes