base64 decoding displays sentinel instead of incorrect character

It is mainly commit 70c83b3. It fixes one bug and a minor problem:
- during base64 decoding, in the main loop, the error message displays the sentinel instead of the offending char
- makes base64lookup table Uint8 instead of Uint

The second commit is because the code retouching went a bit out of control.

Disclaimer: I have never done a PR, so my apologies if I am doing this wrong. And I am new to Julia too...