Make the I Tried star appear when a top-level page load fails for networ...

...k-related reasons.

Under the hood, this requires treating the I Tried pipeline as a new load instead of a replacement, since the failure-handling code interacts poorly with the rest of the replacement code when we get a series of staggered failures over time from the various pipeline components.

The changes in Window work around an underlying Rust runtime problem where the timer controller task doesn't ever notice that its script task owner has failed, so Servo would otherwise hang on shutdown after showing an I Tried star.