Context

The error monad has evolved a lot. It's time to write a new introduction/tutorial for developers.

WIP

Manually testing the MR

Checklist

Document the interface of any function added or modified (see the coding guidelines)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
Select suitable reviewers using the Reviewers field below.

Edited Mar 01, 2021 by Raphaël Proust