I've added a prebuild event command to my project solution that calls the .less (dotless) compiler to compile my .less files:
"$(SolutionDir)packages\dotless.1.4.0.0\tool\dotless.compiler.exe" -m "$(ProjectDir)Assets\Style\source\bootstrap\bootstrap.less" "$(ProjectDir)Assets\Style\css\bootstrap.css"
When I run the command in the Windows command prompt, it runs fine and the .css file gets builded. However, when I build the solution, Visual Studio throws an error:
... exited with code 9009
Which seems to be a generic error telling 'something is wrong'. Paths are correct, the prebuild environment variables can correctly be resolved.