Function parameter name string does not necessarily exist, so it's
better to use the function name as the token in the error message.

BUG=chromium:784158
TEST=angle_unittests

Change-Id: I8f3b8604fd702bdc9486b8d721a5f60de1ff3fa7
Reviewed-on: https://chromium-review.googlesource.com/765972Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>