Error handling & backend polishing PR 1: Applications.py

I though that it would be better to do it in parts. I'll be reviewing one by one the `*.py` files trying to handle possible errors and doing some polishing if I can.

This first PR is for `applications.py` where I can't see any clear case where something can go wrong. The JSON files will be just empty if no info is found.

I unified the calls `/api/v1/applications` and `/api/v1/samples_applications` in one and removed one handler. Those calls are always called together and one call and one handler is better than 2, don't you think?

Nice weekend!