Add 404 and 500 error handling

[ankush]

Add 404 and 500 error handling.
Add simple 404.html template which can be improved later.

related to #46

[BlairCurrey]

Any suggestion how to test 500 error handling? Trying to figure out a way atm.

[ankush]

One example: writing a very large review will crash Body parser and crash server IIRC. That would normally give you JS error stack, now it will only show 500 error to users.
You can still see stack in console.

[BlairCurrey]

OK, in order to test for a 500 error I added throw new Exception(); to a route and used it. The 500 error handling works.