FLEDGE: Remove SellerWorklet::ScoreAdResult.

Instead, pass a score and error messages directly to the passed in
callback. Also make error messages a vector, to allow for multiple
errors.

A 0 score indicates an error or no bid - Mojo doesn't allow Optional
primitive types, so don't use an optional for that. The value is
currently not passed over Mojo, but the worklet API will soon be
converted to a Mojo API.

Bug: 1205517
Change-Id: I745bed9cfc04c9bf877356ffe35dff2568f9ed8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2879875


Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880991}