ikiwiki hosting internals/ bugs/ site creation failure not noticed

Since site creation over the web happens in the background, there's a problem in detecting when the site creation fails.

Instead of a useful error message, the user will be dumped into a site that failed to be created, or branched, and is in some unknown broken state.

The code currently just checks that the site exists, and is not locked, and assumed that means ikiwiki set it up correctly. There should at least be some positive sign checked that ikiwiki finished successfully.

> done; nonce checked