Prints more info when a particular file is not found.

Bug: angleproject:3333
Change-Id: I11ceb0d319023cd1fd23fd25297e5367030342a0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1550899Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>