[roll] Roll fuchsia [shac][gn] Print files that gn fails to parse when formatting

This change fixes `fx format-code` so that GN files with syntax errors will be identified in the error output, instead of a error with no filename attached.

It now collects all syntax errors during the fallback checks and raises a fatal Starlark error (`fail()`) at the end of the check. This ensures that `fx format-code` (which runs `shac fmt`) correctly fails locally, while still allowing all errors to be collected and reported together.

The logic has been simplified and unified to remove redundant branching on the `gn format --dry-run` exit code (which is order-dependent when both formatting changes and syntax errors are present). Instead, we always parse stdout to identify clean unformatted files, and fall back to individual `--stdin` runs for any remaining files if any parsing errors were detected.

TAG=agy

CONV=8c3b0407-8c3f-40b5-8f28-13755aed5a4f

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1620194
Original-Original-Revision: ffc585dcf8e816bbef6b35a99cf1b88b8d3712ee
GitOrigin-RevId: 71ebb3804fe25d15f7dc1aadf35465902691471a
Change-Id: Iaee2c00161152d80d71e0e8bc92fddef60546ab3
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1627480
Cr-Commit-Position: refs/heads/main@{#193449}