Inform user on why a known shaft trap isn't triggered (fixes bug 2294949).

Be more agressive about making shaft traps on invalid levels disappear in a puff of logic.

If a wizard command creates a trap shaft on an invalid level, inform the user.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7536 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on November 22, 2008
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