patch applied (cabal-install): Fix #490, unpack now gives a proper error message.

[Duncan Coutts]

Sun Feb  8 08:52:40 PST 2009  Andrea Vezzosi <sanzhiyan at gmail.com>
  * Fix #490, unpack now gives a proper error message.
  Ignore-this: 358dd291624f8858a52ae2ff27a7e0c2

    M ./Distribution/Client/Unpack.hs -12 +24

View patch online:
http://darcs.haskell.org/cabal-install/_darcs/patches/20090208165240-cd449-d8fc4548b5cb8fd22ba7cf437e362f8d68cba662.gz