Deprecate the KSslError::Error ctor of KSslError

Summary:
This encourages to use the QSslError ctor, which has no loss of information
and will be straightforward to port to QSslError entirely eventually.

There's one more user left in KIO, that requires a resolution of the human
readable error string question in T11620.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11620

Differential Revision: https://phabricator.kde.org/D23920