Extend exception/cert handling

related to #28 
@jhass : I talked with the security team of Mageia tonight. Non-UTF-8 isn't a bug. Fedora works the same (it's packaged differently, but file says us-ascii on Fedora and ISO-8859-1 on Mageia). So the problem might be present on every distro of the RedHad family. I quote :

> Augier: it's packaged differently on Fedora, but the file command says charset=us-ascii on Fedora, and charset=iso-8859-1 on Mageia, so it's not utf-8 in either case
>  Augier: it's a PEM file in either case though
> only difference really is ours has both human-readable information and raw certs, theirs just has the raw certs

No more informations.