Make extra-sure to reject invalid mutations. And if the player somehow

gets an invalid one anyways (a mutation for which mutation_name() has no name) then when listing abilities give a line with error information instead of a blank line.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5751 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on June 12, 2008
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