Incorrect/incomplete version-check JSON can mess up ACP

Many extensions use the new version-check feature to allow admins to see if the extensions are up-to-date.

But I think the actual parsing of those version JSON files might be problematic.

If for instance, there is no "announcement" line in the JSON, the ACP will produce a PHP warning:

[phpBB Debug] PHP Notice: in file [ROOT]/phpbb/version_helper.php on line 274: Undefined index: announcement

This happens when viewing "details" of an extension in the ACP, but also when doing a "Re-Check all versions" action.

Worse, if Gzip is enabled, this will actually produce a "Content Encoding Error".

Basically, it's able to mess up someone's ACP, by simply having one extension which doesn't play by the rules.