As per specification, https://wicg.github.io/background-fetch/#dom-backgroundfetchmanager-fetch does not reject if individual record fetch fail.
This seems to indicate that CSP, port checks and so on should not trigger rejection of https://wicg.github.io/background-fetch/#dom-backgroundfetchmanager-fetch, but would trigger rejection of individual https://wicg.github.io/background-fetch/#dom-backgroundfetchrecord-responseready.

This does not seem to align with current WPT tests, see for instance mixed-content-and-allowed-schemes.https.window.js, port-blocking.https.window.js or content-security-policy.https.window.js.

We should either update the spec or update the tests.
From a spec point of view, it seems easier to stick with calling fetch as is.
From a WebKit implementation point of view, it seems also easier to stick with the spec.