improve error message
improve error message

Ignore-this: e90f98c675cc6df2770c8bba3a7c313

darcs-hash:20130316130734-1a7c6-5e34d40efe9979f39d53361dbd0c194b1d050af3

jimcrayne/jhc,jimcrayne/jhc,jimcrayne/jhc,jimcrayne/jhc