Always attach some URL/target data point to fetch/xhr/request errors

### Description

The Sentry Chief Product Officer has brought to our attention that Fetch errors sometimes suck. ([example that will expire in 90 days](https://peated.sentry.io/issues/5656229682/?referrer=regression_activity-email¬ification_uuid=add413c0-019a-4b87-ae37-608eb905a605))


We technically have a stack trace here but sourcemaps are broken so the user has basically no idea what fetch request failed. It's very hard to gauge from breadcrumbs what exact request failed.

Side note, the fetch call that failed looked like `fetch("")`. We render this as in the product, you guessed it `""`. Which is crap because it actually means the request went to the current route. We should at least coerce that string to `URL(fetchTarget, window.location.href).toString()` in the SDK.