font/sfnt: fix dropped errors
font/sfnt: fix dropped errors

Pick up dropped errors in parseGPOSScriptFeatures()
and parseGPOSKern().

Change-Id: I50d9b1feca4c69163df644702afb0c386c2987f7
Reviewed-on: https://go-review.googlesource.com/c/image/+/210499
Reviewed-by: Sebastien Binet <e007d926a61a64a3456bccd7b023e55740ae3e6a@gmail.com>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/image