- Detect errors during generation of compile_commands.json
  and building the target 'v8_generated_cc_files'.

- Change format of JSON output to have files as keys, which makes
  programmatic lookup by filename easier on the consumer side.

Change-Id: Ibc3d9cff64f82df7a3dbb76cb8d914b29460a48c
Notry: true
Reviewed-on: https://chromium-review.googlesource.com/c/1362041Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58263}