Revert "convert fail to error"

This reverts commit d14770ca9c.

That changed the type of error from an IOError to something else, so broke
stuff that was catching IOErrors.

So back to a UserError, but be explicit this time that's what it's
throwing.

Utility/Process.hs

@@ -133,7 +133,8 @@ forceSuccessProcess p pid = do
 	code <- waitForProcess pid
 	case code of
 		ExitSuccess -> return ()
-		ExitFailure n -> error $ showCmd p ++ " exited " ++ show n
+		ExitFailure n -> ioError $ userError $
+			showCmd p ++ " exited " ++ show n
 
 -- | Waits for a ProcessHandle and returns True if it exited successfully.
 -- Note that using this with createProcessChecked will throw away