Centralize error handling

[n-dusan]

Have all the exit process calls be stored in a single, centralized location.

This is a minor point, but generally we only want one central call to `exit()` in a program. 
The idea being that we bubble up all errors with `?` and only at some common overarching point 
in the code do we decide to exit based on the received error. 
This can get hard with threads and async, etc. But ideally, 
centralising the `exit()` can help us avoid some unexpected bugs and 
generally improive architecture. But don't worry about it too much, it's just something to bear in mind.

Originally posted by @tombh in #33 (comment)