Unconditionally check for errors (instead of a DCHECK), and print a
better error message (including the file path) if the file cannot be
opened.

R=tebbi@chromium.org
CC=machenbach@chromium.org

Bug: chromium:1087039
Change-Id: Ia5cb76b309e78631ecf9462de6c7cd3eb4bf9e59
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2224226Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68097}