convert/svn: abort operation when python bindings are not available Subversion python bindings check was not present in svn_sink source class which made it fail while using svn as destination repository. Added a more maintainble svn bindings check for svn_source and svn_sink classes.

convert/svn: handle MissingTool exception from converter_sink class The converter_sink class had no way of handling MissingTool exception. This change makes it to abort when there is a MissingTool exception.

bookmarks: restrict moving a bookmark to its descendants (issue1502) A bookmark can only move to a descendant on commit, pull or unbundle. Bookmarks cannot jump between heads anymore. This fixese issue 1502. We explicitly use new.node(), to emphasise that we are updating the current bookmark to the new node.