THRIFT-2470: Stop THttpHandler from swallowing processor exceptions

The problem with the way it works currently is that it makes it very difficult to figure out what's going on when I've got a handler running on a remote web server. If the handler just allows the exceptions to bubble up like normal, they get caught by our normal exception handling code and logged.

I also added a new project for unit tests, and some tests for the changes I've made.