Check if update_core() exists before calling it in Core_Upgrader

[nacin]

If we've successfully required() wp-admin/includes/update-core.php, but the update_core() function does not exist, it's fairly obvious that we're dealing with a disk quota issue — the file is there, but empty.

While md5 verification of files (#18201) will catch this earlier, that is not necessarily scoped for 3.4, and this is an easy win with a targeted error message that may be better than what #18201 can provide.

There aren't a whole lot of Google results for this error message, but a friend just ran into it as I egged him to update, and the chances of that happening is enough for me to think that it catches quite a few users. ​https://www.google.com/search?q=fatal+error+update_core

Patch attached. If we want to make it simpler, we can just return the generic copy_failed message, no new string.

[nacin]

In [20619]:

Check if update_core() exists before calling it in Core_Upgrader. If not, issue an error message that the copy failed likely due to disk space. fixes #19883.