Allow the response code to be passed as a shorthand to the `$title` or `$args` parameter of `wp_die()`, for brevity.

See #10551 and #11286
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Switch to a `403` response code in places where it is more appropriate than a `500` due to permissions errors.

Fixes #10551
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Built from https://develop.svn.wordpress.org/trunk@30356


git-svn-id: http://core.svn.wordpress.org/trunk@30355 1a063a9b-81f0-0310-95a4-ce76da25c4cd