update-core.php doesn't remove the upgrade files if the upgrade fails due to too old of a PHP or MySQL version

[nacin]

Related: #11464, where this code came from.

wp-admin/includes/update-core.php, which gets copied over after download during an upgrade, has a new check in 2.9 to ensure that the MySQL and PHP versions are is required by the downloaded version.

However, I didn't account for the fact that the code should clean up after itself by deleting the downloaded package on failure, just as it does on any other failure it encounters. This should be fixed in 2.9.1.

Patch on its way.

[nacin]

We also need to bump the local $wp_version to 2.9.1. See also #11639.

[ryan]

(In [12564]) Cleanup after bailing when php or mysql versions are not compat. Props nacin. fixes #11638 for trunk

[ryan]

(In [12565]) Cleanup after bailing when php or mysql versions are not compat. Props nacin. fixes #11638 for 2.9