LibJS: Fix Promise constructor reject function argument

If calling the executor function throws an exception, the return value
of `vm.call()` will be an empty value, which we then passed as an
argument to the reject function, which is incorrect - what it actually
needs is the exception value. This stems from a misunderstanding of the
spec I had at the time of implementing this - in their case, the
exception value is part of the completion record returned by Call().

This error was previously masked as we would use a fallback
(`value_or(js_undefined())` for the empty value argument, but that was
removed in 57f7e6e.

Fixes #8447.

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