Catch a KeyError when trying to remove a non-existent herald.
Catch a KeyError when trying to remove a non-existent herald.

jeffmahoney/supybot,ProgVal/Limnoria-test,mazaclub/mazabot-core,ProgVal/Limnoria-test,raboof/supybot,haxwithaxe/supybot,Ban3/Limnoria,mazaclub/mazabot-core,buildbot/supybot,kblin/supybot-gsoc,frumiousbandersnatch/supybot-code,Ban3/Limnoria,prashantpawar/supybot-rothbot