Make messages for animating single skeletons/corpses versus multiple

ones consistent. Also, display a proper warning message if a player tries to animate a single skeleton/corpse while not actually standing on one.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8707 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on January 22, 2009
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