This avoids unnecessary pool allocations when generating errors. Most
of the returned strings are static.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ia9b26898d499985d61e629fddde7aded4714eddf
Reviewed-on: https://chromium-review.googlesource.com/885818Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>