cabal run: "Preprocessing executable" message shouldn't be in stdout

The `Preprocessing executable 'X'...` message is currently written to stdout, even for `cabal run`. This contaminates `stdout`, so `cabal run` cannot be used with any program that outputs program-readable through `stdout` (e.g. Pandoc filters).

```
$ cabal run 2>/dev/null
Preprocessing executable 'X' for
Y...
Running X...
^C
$ cabal --version
cabal-install version 1.24.0.2
compiled using version 1.24.2.0 of the Cabal library
```