stderr redirected to stdout

If you look to logs build logs ...
https://ci.appveyor.com/project/jwood803/xfsharp/build/1.0.16
you will see that 
```
Status:    Ok
```
So the FAKE script successfully run and `Example.dll` was generated.

BUT! `AppVeyor` and `PowerShell ISE` think that script failed.
I think that it happens because `FCS` write something to `stderr` stream during source code compilation.
PS thinks that executed app failed and fails current script with `RemoteException`

As a workaroud I redirected `stderr` to `stdout` and it works OK in my `PowerShell ISE`.
I think that this `FCS` issue https://github.com/fsharp/FSharp.Compiler.Service/issues/201 is related.
When we will remove `stderr` messages from `FSC` it should resolve this issue as well.