Further unified error handling between SkSL and DSL

This eliminates the SkSL ErrorReporter class and funnels everything
through the DSL ErrorHandler. Since the DSL error handler can be
changed, this required a number of updates to ensure that things work
properly in the face of custom error handlers. There is probably more
work to be done in that area, but this at least passes all existing
tests.

Change-Id: Iaee27b79fc4ed426c484ccab257c09d28619ead5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438116
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
38 files changed