Make base32 handling less forgiving

Ordinarily, one only encounters base32 strings of length 0, 2, 4, 5, or
7 characters (mod 8). If it is of length 3 or 6, the final character is
in the middle of an incomplete byte. As with length 1, this should
probably throw an error, as the other alternative is to entirely ignore
the character.

(It already did throw an error before this change, but
unintentionally.)