`wikierrors` is not exhaustive (see http://www.mediawiki.org/wiki/API:Account_creation "Possible Errors" section).

Also, even if the list is updated to be exhaustive, it still isn't future-proof.  If MediaWiki adds/changes error codes, the WIKI_SIDE_FAILED error will not be populated.  I'd make the following changes:

- [ ] add WIKI_SIDE_FAILED error regardless
- [ ] notify devs if the error received is not in the list