Fix the error handling and validation framework

[axymthr]

The current error handling and user input validation framework is conceptually confusing.
We have a MVC application so the error handling should follow appropriate Spring MVC style error handling. The current code follows. RESTful style and this muddies things.
One of the major issues is the 500 error response, which is JTE based. Unless we need to render dynamic information in the 500 response, there's no reason to render a static JTE template. This blows up if template compilation is incomplete, and goes into a recursive stack overflow state on page load. This is the wrong way to do error handling for web views.