Prior to about 2013, many class methods lacked even access modifiers which made the `@access` notations that much more useful. Now that we've gotten to a point where the codebase is more mature from a maintenance perspective and we can finally remove these notations. Notable exceptions to this change include standalone functions notated as private as well as some classes still considered to represent "private" APIs.
See #41452.
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An upgrader class is used in conjunction with an upgrader skin class. A skin class handles the logging for an upgrade and informs a user about the progress and failures.
The current Ajax install/update handlers are using the `Automatic_Upgrader_Skin` class because during an Ajax request no output is intended. The difference between Ajax updates and automatic updates is that you will see the full log (usually by email) while Ajax updates focus only on success or failure. For that `Automatic_Upgrader_Skin` has one disadvantage: It doesn't provide a way to retrieve failure messages which were passed through `WP_Upgrader_Skin::error()` by the upgrader.
To solve this issue a new skin `WP_Ajax_Upgrader_Skin` has been introduced. The skin extends `Automatic_Upgrader_Skin` and overrides the `error()` and `feedback()` methods to intercept all errors, which can be a `WP_Error` object or a string.
This updates all four Ajax handler for installing/updating themes/plugins to use the new skin. They now also check the skin for any intercepted errors and pass them on to the user.
Props flixos90, obenland, ocean90.
Props DrewAPicture, pento for review.
Fixes#37531.
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