Check (and fix if necessary) parsing of invalid URLs in servlets about app-inventor HOT 1 CLOSED

I checked that using an ill-formed URL (e.g., /ode/download or 
/ode/download/file/12) does not crash the production server (it catches the 
error and returns a 404). For the App Engine based servers, the user will see a 
stack trace. Since neither of these things is dangerous, we'll hold off on 
actually fixing this for now since it isn't a high priority. The right way to 
fix it is to catch the possible errors in the servlet code and return a 400 
status code (Bad Request).