Add access modifiers to methods/members in `WP_Error`. Add a magic `__get()` method for BC.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28511


git-svn-id: http://core.svn.wordpress.org/trunk@28337 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add access modifiers to methods/members in `WP_HTTP_IXR_Client`.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28512


git-svn-id: http://core.svn.wordpress.org/trunk@28338 1a063a9b-81f0-0310-95a4-ce76da25c4cd