fixed a bug for sequential models with only one value of the sequential parameter

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/caret@840 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
fixed a bug for default value of bagControl() and added error checking

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/caret@841 edb9625f-4e0d-4859-8d74-9fd3b1da38cb