Original commit message:

    [serializer] allow SnapshotCreator to destruct without a blob

    Previously SnapshotCreator demanded a blob to be created before
    it can be destructed in debug build, this patch removes the
    DCHECK so that the embedder can choose not to create the blob
    when e.g. the snapshot building isn't successful due to errors.

    Change-Id: I72939be1e0d79b257b9761f48a72e45325a1f6d8
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3716682
    Reviewed-by: Camillo Bruni <[email protected]>
    Commit-Queue: Joyee Cheung <[email protected]>
    Cr-Commit-Position: refs/heads/main@{#81644}

Refs: v8/v8@f3cad8c

This patch refactors the SnapshotBuilder::Generate() routines
so that when running into errors during the snapshot building
process, they can exit gracefully by printing the error
and return a non-zero exit code. If the error is likely to
be caused by internal scripts, the return code would be 12,
if the error is caused by user scripts the return code would
be 1. In addition this refactors the generation of embedded
snapshots and directly writes to the output file stream
instead of producing an intermediate string with string
streams.