Possible fix for bug 2293518: if a monster tries to unsubmerge while there's

already a different monster on the same square, then treat it like a monster trying to unsubmerge from underneath the player. Also give an error message (with a reference to the bug number) since two monsters aren't supposed to be on the same square.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7532 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on November 22, 2008
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