Field validation against MediaWiki.org for extension name

[mattflaschen]

If Extension:ExtensionName is taken (as determined by checking MediaWiki.org API with JSONP), show an error on the field and don't let them download.

[edg2s]

Is this possible or even correct? A simple popular extension name may be being used by old abandoned code. We should definitely show a warning if their chosen name is known about, but probably not a fatal error.

[mattflaschen]

There could be some kind of override, but I would argue this is rarely a good idea. It's not that hard to find a non-colliding name, and save people a lot of trouble down the road.