patch applied (packages/unix): Throw a proper exception if getGroupEntryForName fails to find an entry

[Ian Lynagh]

Sat Nov 10 15:58:05 PST 2007  Ian Lynagh <igloo at earth.li>
  * Throw a proper exception if getGroupEntryForName fails to find an entry
  We used to get
   *** Exception: getGroupEntryForName: failed (Success)
  Fixes trac #1655

    M ./System/Posix/User.hsc -6 +15
    M ./tests/all.T +2
    A ./tests/getGroupEntryForName.hs
    A ./tests/getGroupEntryForName.stderr