improve svn support in catkin_prepare_release

The commit message should contain the tag name - not only the version number.

SVN should check earlier if the tagging structure is supported (or not) to not do changes if it will fail later anyway.

Besides support for tagging trunk or a specific branch:

*  allow tagging arbitrary subfolders but then require a tag name prefix

@k-okada Please check if that version suits your needs by trying to do some test releases from your svn repos.