I18N: Restore the "Error:" prefix for error messages.

Partially reverts [48059] as there's no full consensus on the removal of the text prefix. Further actions should be taken to improve consistency and accessibility of the admin notices. Keeps some improvements to the translatable strings from [48059].

Fixes #47656.

Built from https://develop.svn.wordpress.org/trunk@48115


git-svn-id: https://core.svn.wordpress.org/trunk@47884 1a063a9b-81f0-0310-95a4-ce76da25c4cd