MGCB Improvements

Added some fixes to the MonoGame Content Builder tool so it can be a better host for developing and testing improvements to the content pipeline.

 - MGCB now links against MonoGame and not MS XNA.
 - Added some initial exception and error handling which prints errors to the console.
 - Removed the MSBuild Tasks from the pipeline project (didn't delete the files yet).
 - Fixed some random pipeline bugs.

With these changes everyone should be able to work out of the MGCB solution for doing content pipeline development work.