Accessibility: Text Changes: Use sentence case for the word `Error` in various...

Accessibility: Text Changes: Use sentence case for the word `Error` in various error messages, instead of all caps.

Using all caps should be avoided for better readability and because screen readers may pronounce all-caps words as abbreviations.

Props afercia, ryokuhi, sabernhardt, garrett-eclipse.
See #47656, #43037, #42945.
Built from https://develop.svn.wordpress.org/trunk@47156


git-svn-id: https://core.svn.wordpress.org/trunk@46956 1a063a9b-81f0-0310-95a4-ce76da25c4cd