Remove ambiguous import of 'catch'

Running 'cabal install Cabal/ cabal-install/' from the latest git version resulted in:

Distribution/Client/Compat/Process.hs:41:5:
    Ambiguous occurrence `catch'
    It could refer to either `Prelude.catch',
                             imported from `Prelude' at Distribution/Client/Compat/Process.hs:16:8-41
                             (and originally defined in `System.IO.Error')
                          or `Control.Exception.catch',
                             imported from `Control.Exception' at Distribution/Client/Compat/Process.hs:20:37-41
                             (and originally defined in `Control.Exception.Base')


This pull requests fixes that.