Fixed error reporting on invalid SkSL assignments

Change-Id: I9859081a14b110731f943e09fdd94dc10e0c9dfc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/353580
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed