Fix encoding problems with 7z archives.

From: Benoit Pierre <benoit.pierre@gmail.com>

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1042 ef8490bc-f317-4725-81ed-e95a9aca1bcf
More descriptive error messages.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1043 ef8490bc-f317-4725-81ed-e95a9aca1bcf