Fix MSBUILD script

MSBUILD has default message parsing

[MSBuild / Visual Studio aware error messages and message formats](http://blogs.msdn.com/b/msbuild/archive/2006/11/03/msbuild-visual-studio-aware-error-messages-and-message-formats.aspx)

Adding IgnoreStandardErrorWarningFormat="true" to the [Exec](http://msdn.microsoft.com/en-au/library/x8zx72cd.aspx) statement disables this behaviour

This resolves the [behavior](https://github.com/sawilde/opencover/issues/90) that was mistakenly attributed to [OpenCover](https://github.com/sawilde/opencover)