[css-view-transitions-1] Clarify what "return failure" means

[nt1m]

There are 3 occurrences of "return failure" in the spec. Can we clarify what they mean? Do they throw a JS exception, a console message or just an early return?

[nt1m]

cc @khushalsagar @noamr @vmpstr @fantasai @tabatkins

[noamr]

We have "If failure is returned" in other algorithms which take this into account.
So it's an early return, and invoke whatever next step in the calling algorithm that depends on the return value being a failure.

[nt1m]

Ah I missed that, thanks!