Have heartbeat's connection lost notice handle 503 errors send by the upgrader's maintenance mode.

fixes #25660 for 3.7.

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3.7-RC2

Built from https://develop.svn.wordpress.org/trunk@25875


git-svn-id: http://core.svn.wordpress.org/trunk@25875 1a063a9b-81f0-0310-95a4-ce76da25c4cd