marketstat/json does not always return valid JSON

[Ionaru]

The marketstat API endpoint in JSON format does not always return valid JSON, for instance when an invalid typeid is given.

https://api.eve-central.com/api/marketstat/json?typeid=123456 for example returns "A non-marketable type was given, or some other error occurred.". This return can not be parsed as JSON, and will break code that expects to return to be always JSON.