Make non-ignorable SSL errors explicit

Summary:
So far this is done implicitly in KSSLD by not being able to persist
these error codes in its config file. This however is rather error
prone, and requires manual enum <-> string mapping there.

Now we do this explicitly, which allows us to eventually use QMetaEnum
for serialization in KSSLD.

Reviewers: Frameworks, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11620

Differential Revision: https://phabricator.kde.org/D24742