When user profile isn't found, throw a 404 rather than 500

When visiting a page where the user doesn't exist, example link: 
https://courses.edx.org/courses/MITx/6.00x/2013_Spring/discussion/forum/users/0 
You will see a 500 error instead of a 404. This handles that. Addresses https://edx-wiki.atlassian.net/browse/LMS-513

@wedaly can you help me figure out how I would test this? The forums generally don't seem to have great unit tests here.

@adampalay