patch applied (packages/Cabal): Tweak flags error message and use monad failure rather than error

[Ian Lynagh]

Sat Jan 26 16:11:37 PST 2008  Duncan Coutts <duncan at haskell.org>
  * Tweak flags error message and use monad failure rather than error

    M ./Distribution/PackageDescription.hs -1 +1

View patch online:
http://darcs.haskell.org/packages/Cabal/_darcs/patches/20080127001137-adfee-a5451e7ddb8a475e37485e3b539c90e45e3b45c2.gz