Error Handling Middleware

[rugulous]

We should probably have some form of custom middleware that deals with any errors that arise

Obviously we shouldn't have any errors in the first place (!), but it would be much better if we can deal with them and at least squeak out a 500 error instead of the whole site just dying

It could even do different things depending on the type of exception raised - validation exceptions, authentication exceptions and sequelize exceptions will all map to different status codes, and we could perhaps even drill deeper!