If the `wbformatvalue` api is called with the string `unknown` for the type, then an Exception is thrown and an internal server error is logged. See the parent ticket for more information.
* Exception with `{"value":"123","type":"unknown"}`: https://www.wikidata.org/wiki/Special:ApiSandbox#action=wbformatvalue&format=json&datavalue=%7B%22value%22%3A%22123%22%2C%22type%22%3A%22unknown%22%7D
** The error message is `lib/includes/Formatters/DispatchingValueFormatter.php: No formatter defined for value type unknown.`
** See parent ticket for the stack trace
* The expected error message for `{"value":"123","type":"I do not exist"}`: https://www.wikidata.org/wiki/Special:ApiSandbox#action=wbformatvalue&format=json&datavalue=%7B%22value%22%3A%22123%22%2C%22type%22%3A%22I%20do%20not%20exist%22%7D (though that could still be worded better than "An illegal set of parameters have been used.")
**Acceptance criteria**
* [ ] Calling `wbformatvalue` with `{"value":"123","type":"unknown"}` results in the usual error response for a non-existing type and no Exception is thrown but not caught