[sksl] Raise feature errors before code generation

Change-Id: Icb7a4e18349eecc73f01501b960893ebeaead1b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755239
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed