Improved error reporting in DSLParser::swizzle

With this change, it will either match or improve on the reports we
currently get from SkSLParser.

Change-Id: I9b3c0f0c2225bf47fec141a1c01c94d9c2ab6a6b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443056
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed