refactor: centralize error messages

[steve-chavez]

Problem

Error messages are scattered everywhere. This hurts maintainability.

Solution

Centralize error messages into a single module.

I've used the same technique done in PostgREST: an observation type plus a callback that acts upon it.

Notes

I also tried to make the functions pure (and return an error type) but this is more cumbersome because some of extra parameters would need to be allocated to be passed.

At least non-pure functions now have a "marker" (explicit observation handler as a parameter).