Options, Meta APIs: Improve error handling in sanitize_option().

To prevent potential false negatives, set `$error` to `null` initially, so we can better tell if it was ever changed during the sanitization and be able to better react if an empty string is added to it.

Additionally, and mainly for the sake of the Settings API at this point, add error messages to some `WP_Error` objects returned from `wpdb` methods that were previously causing the issues here.

Follow-up to [32791].

Props iCaleb, audrasjb, hellofromTonya, SergeyBiryukov.
Fixes #53986.

git-svn-id: https://develop.svn.wordpress.org/trunk@52294 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/option/sanitize-option.php

@@ -155,6 +155,7 @@ class Tests_Sanitize_Option extends WP_UnitTestCase {
 			array( '/%postname%/', '/%postname%/', true ),
 			array( '/%year%/%monthnum%/%day%/%postname%/', '/%year%/%monthnum%/%day%/%postname%/', true ),
 			array( '/%year/%postname%/', '/%year/%postname%/', true ),
+			array( new WP_Error( 'wpdb_get_table_charset_failure' ), false, false ), // ticket 53986
 		);
 	}