This allows us to return the Sync's label when validation fails for a
Sync related entry point.

BUG=angleproject:3643

Change-Id: I64e8a8855a2e3b51768c2b560fdc4696bb325351
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1687120
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>