implemented treat warning as errors commandline option.

Same name as csc.exe uses: [msdn](http://msdn.microsoft.com/en-us/library/406xhdz3.aspx)
Issue #828

Feedback appreciated but feel free to close when you don't want this functionality at this point :)
I'm curious, do you actually get an empty sourcemap file if you don't perform this check?

Sorry, I'm personally not familiar enough with the harness code.