enforce result checking for s2n_stuffer methods

[jmayclin]

Problem:

We currently rely on contributors to remember to check all of the return values from s2n_stuffer method. Relying on people to remember this is no ideal because people can forget, and it creates extra friction in PR comments.

Solution:

Similar to #4389, we can enforce that the return values from s2n_stuffer methods are used. This way customers will receive an error message if they forget to check the return value.

Requirements / Acceptance Criteria:

Customers must receive an error when s2n_stuffer return values are not checked.