Use guint8 for 8-bit unsigned numeric values.

That slightly better expresses the intent.

Also, fix the message printed for bad bytes in a field name to handle
bytes that are bad because they don't correspond to printable ASCII
characters.

Change-Id: I01f232c35bbbe30286999b6c607bfcf3d491453d
Reviewed-on: https://code.wireshark.org/review/4976
Reviewed-by: Guy Harris <guy@alum.mit.edu>