Improve fetch failure error reporting

[emmahoggan]

Context

We were inadvertently squashing fetch failure details. This makes improvements to the error display including:

• not assuming status/body structure - instead formatting leads with the message (always present), appends status only when set, and appends the body only when present
• prints the fetch failure 'cause' if available - Node's top level fetch error just presents 'fetch failure', so we now walk the cause chain, append each code, and drops the uninformative "fetch failed" wrapper
• non-JSON 2xx responses carried no status or body - we now pass the real status and the raw response text, distinguish an empty body, and name the URL

[andrewjensen]

🔥🔥🔥