Output: Use throwError from Control.Monad.Except

Update 'Idris/Output.hs' to use 'throwError' from 'Control.Monad.Except'
rather than from 'Control.Monad.Error', which causes the build to fail
with a default setup:

  src/Idris/Output.hs:18:1: Warning:
      Module ‘Control.Monad.Error’ is deprecated:
        Use Control.Monad.Except instead

  <no location info>:
  Failing due to -Werror.

With this patch it should build fine.

This requires mtl >= 2.2.1, so add this to the cabal file.

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>