Interpreter: Better error message for unknown operators

It's possible to hit this with the comma operator. Not sure if there are
any others.

Change-Id: I10fb68d7a20ce54fccc1597da8e656fa7d61cd97
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230197
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed