Instead pass error code and error message directly to the Context. As
a side effect every validation error has an associated message now.

Reduces binary size by almost 100k.

Bug: angleproject:2957
Change-Id: I045e8ab57f8f9d787654a32f6f037011fe3a9f24
Reviewed-on: https://chromium-review.googlesource.com/c/1335890
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>