Comments: Make `wp_update_comment()` return `false` instead of 0 for an invalid comment or post ID.

This addresses an inconsistency where 0 could mean one of the three scenarios:

* Invalid comment ID.
* Invalid comment post ID.
* No DB rows updated. This is not an error and should not be treated as one.

With this change, `wp_update_comment()` always returns either `false` or a `WP_Error` object on failure, depending on the value of the `$wp_error` parameter.

Follow-up to [48154], [48215], [48216], [48218], [48230].

Props dd32, jnylen0, enrico.sorcinelli.
Fixes #39732. See #38700, #39735.
Built from https://develop.svn.wordpress.org/trunk@48235


git-svn-id: https://core.svn.wordpress.org/trunk@48004 1a063a9b-81f0-0310-95a4-ce76da25c4cd