Make KSslError hold a QSslError internally

Summary:
This implies a bit back and forth conversion for now, but it enables
migrating the code using KSslError to use QSslError instead, without
breaking API compatibility. Using QSslError gives us a greater level
of detail for the error messages, and enables deprecating KSslError.

Reviewers: dfaure, aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11620

Differential Revision: https://phabricator.kde.org/D23919