print better error message if user is confused about tag vs. branch

the xcvs-taginfo script already has a notion of a list of regular expressions that are valid tags vs. valid branches. as people try to use the new release system, they're already getting confused about what should be a tag, and what should be a branch. it'd be nice that if the requested branch is a valid tag, but not a valid branch, or if they're trying to add a tag that would be a valid branch, we could print something more descriptive. something like:

• invalid branch:
  "ERROR: %s is a valid name for a tag, but not a branch. You must either remove the -b option to your cvs tag command if you do not want to create a branch, or choose a valid branch name. See http://drupal.org/handbook/page/here for info about valid branch and tag names."
• invalid tag:
  "ERROR: %s is a valid name for a branch, but not a tag. You must either use the -b option if to your cvs tag command if you wish to create a branch, or choose a valid tag name. See http://drupal.org/handbook/page/here for info about valid branch and tag names."

should be quite easy.