The text of error messages in the UI is too technical. We need to improve them.
More patches for review:
* [x] Easier to understand timeout messages: https://gerrit.wikimedia.org/r/316534
* [x] Specific message for unknown language codes: https://gerrit.wikimedia.org/r/316535
* [x] Replace ugly "illegal value" message on Commons media values: https://gerrit.wikimedia.org/r/316575
* [x] Improve a message on Special:NewItem: https://gerrit.wikimedia.org/r/316537
* [ ] When a sitelink edit fails, the message is extremely vague ("The specified article could not be found on the corresponding site"). It should include site and page. This requires an update of the Wikibase JavaScript API component:
* [x] Make RepoApiError aware of parameters: https://gerrit.wikimedia.org/r/316533
* [ ] Update release notes
* [ ] Tag new version
* [ ] Update Wikibase.git to use new version
* [ ] Add parameters to the message: https://gerrit.wikimedia.org/r/316538