add a couple of more explicit error messages for silly usages

git-svn-id: https://svn.r-project.org/R/trunk@63257 00db46b3-68df-0310-9c12-caf00c1e9a41
add1.glm double df divide

git-svn-id: https://svn.r-project.org/R/trunk@63258 00db46b3-68df-0310-9c12-caf00c1e9a41