translator: Fail compilation if precision emu unsupported. The fuzzer uncovered a case where we were trying to emulate precision on HLSL 3.0, causing an ASSERT crash. BUG=chromium:653276 Change-Id: I2e666a1ff4f605541e25f04264146063559cb835 Reviewed-on: https://chromium-review.googlesource.com/394237 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>