jsonpb: fix a confusing error message

[ijschwabacher]

Because in is a []byte, as written this prints a list of integers instead of the unrecognized enum value.

[dsnet]

Bug fix approved since it fixes a regression relative to v1.3.5. See

protobuf/jsonpb/jsonpb.go

Line 905 in 8466869

return fmt.Errorf("unknown value %q for enum %s", s, prop.Enum)