Add 404 handler for unknown methods of know class

Modify the configuration of the per-class express app to convert
unhandled requests (i.e. unknown sub-urls) into a 404 error.

The change includes code clean-up:
- rename `sc` parameter to `sharedClass`
- use `adapter` instead of `self` in RestAdapter.createHandler
- extract error handling middleware to RestAdapter.errorHandler
- add more comments