The spec tells us to reject the promise with a RuntimeError instead of a
LinkError whenever the module's start function fails during module
instantiation. Fixes 1 WPT subtest in `wasm/core`.
...instead of specially handling JS::Completion.
This makes it possible for LibWeb/LibJS to have full control over how
these things are made, stored, and visited (whenever).
Fixes an issue where we couldn't roundtrip a JS exception through Wasm.
