Perform Discard error-handling in DiscardStatement::Convert.

DSLCore shouldn't be in charge of statement error handling. Errors
should be checked in Convert and asserted-on in Make.

Change-Id: I3f0c2869e5568556f9a367071c2fc622b3478622
Bug: skia:13821
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/595016
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed