Made DSLFunction gracefully handle parameter conversion failure

Previously, a failure in converting a parameter while creating a
function would result in an assertion failure. This fixes it to be a
non-fatal error.

Change-Id: I7a526eff9696847f70f0de78e71e7c23488c1254
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404339
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed