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

wp-admin/import.php

@@ -61,7 +61,7 @@ $parent_file = 'tools.php';
 <h1><?php echo esc_html( $title ); ?></h1>
 <?php if ( ! empty( $_GET['invalid'] ) ) : ?>
 	<div class="error">
-		<p>
+		<p><strong><?php _e( 'Error:' ); ?></strong>
 			<?php
 			/* translators: %s: Importer slug. */
 			printf( __( 'The %s importer is invalid or is not installed.' ), '<strong>' . esc_html( $_GET['invalid'] ) . '</strong>' );