Create tests for the error/failure cases

[claucece]

**Warning! Do not assign yourself this issue yet.**

Why

A library should be sufficiently tested on all cases.

Tasks

• List here the tests needed

Open questions

• Are we freeing and nullifying correctly on each case?

[claucece]

This is something we can tackle at the assembly at CCC. We should create tasks. But this can be a nice start for people in OTRv4.