oathkeygen: modernize and improve error handling slightly

oathkeygen: modernize and improve error handling slightly

Note that this could also just be documented doing:

 dd if=/dev/urandom bs=1 count=10 2>/dev/null | base32

And using TOTP apps that can scan QR codes is much better UX anyway,
but its to trivial to bother deprecating it now and we'd still depend
on libmime-base32-perl, so really nothing gained in removing or
rewriting in shell..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>