patch applied (cabal): Be better about exiting if a command we run fails

[Ian Lynagh]

Tue Apr 17 15:22:57 PDT 2007  Ian Lynagh <igloo at earth.li>
  * Be better about exiting if a command we run fails

    M ./Distribution/PreProcess.hs -27 +19
    M ./Distribution/Program.hs -3 +3
    M ./Distribution/Simple.hs -8 +9
    M ./Distribution/Simple/GHC.hs -12 +10
    M ./Distribution/Simple/SrcDist.hs -19 +7
    M ./Distribution/Simple/Utils.hs -37 +10