Catch cases where adding a book to library fails.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@902 ef8490bc-f317-4725-81ed-e95a9aca1bcf
ALT-LEFT/RIGHT keys respect manga mode setting.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@903 ef8490bc-f317-4725-81ed-e95a9aca1bcf