Plugins and Themes Auto-Updates:

- Fix displaying of errors on the Themes screen.
- Correct the classes used for the error messages, and use `notice-alt` as notices are mostly shown on a white background.

Props afercia, azaozz.
Fixes #50405.
Built from https://develop.svn.wordpress.org/trunk@48092


git-svn-id: https://core.svn.wordpress.org/trunk@47859 1a063a9b-81f0-0310-95a4-ce76da25c4cd