More code path test coverage

Added test coverage for untested code paths (mostly error handling) revealed by 'go tool cover'.
It would be easy to make a comprehensive test here:

    for b := 0; b < 256; b++ {
        n, err := strconv.ParseInt(string(b), 16, 8)
        value, ok := asciiHex(b)
        if err != nil {
             c.Assert(ok, gc.Equals, false)
             c.Assert(value, gc.Equals, 0)
        } else {
             c.Assert(ok, gc.Equals, true)
             c.Assert(value, gc.Equals, n)
        }
    }