[ctioga2] more explicit error on separators not found

git-svn-id: svn+ssh://rubyforge.org/var/svn/ctioga2/trunk/ctioga2@519 86c91514-0d3b-42e0-aed1-4dfce116d947
[ctioga2] round shape for boxes (and, therefore legends too !)

git-svn-id: svn+ssh://rubyforge.org/var/svn/ctioga2/trunk/ctioga2@520 86c91514-0d3b-42e0-aed1-4dfce116d947