If mons_spells() is given an unknown monster spell then assert with the name of

the spell and the name of the casting monster, rather than just "Unknown spell".

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8374 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 9, 2009
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