proto: return more useful error message in SetExtension (#685)
proto: return more useful error message in SetExtension (#685)

Including the types that are mismatched, rather than just stating that there was a type mismatch makes the error easier to debug and fix.
golang/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,golang/protobuf