This is a derived type from gl::Error so makes sense to be there.
Also makes it more accessible than in ProgramImpl.h

BUG=angleproject:1897

Change-Id: Id41b13e5a072745d8c361057f5bef8f152e0452b
Reviewed-on: https://chromium-review.googlesource.com/522872Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>