Reduce log level for IllegalStateException in StaticResourceServlet

The IllegalStateException in StaticResourceServlet occurs primarily as a result of the client closing or refreshing a browser while the server is serving requested files. The server tries to change the 200 response to a 500 (the server assumes, incorrectly, that the issue was because of a server side internal IO error).

The IllegalStateException in StaticResourceServlet is too common as a client side driven issue for the error to be at log level INFO. It should be set to TRACE.