More MGCB Improvements

Even more improvements to the MonoGame content builder:

 - /outputDir and /intermediateDir are no longer required.
 - Better command line usage docs.
 - Improved logging to show exceptions and skipped files.
 - We now throw an exception if the importer doesn't match the processor.
 - Fixed PipelineException constructors.
 - We no longer clear processor parameters on /build.

Still need to finish fixing /clean and add /incremental.