Improvement: marketstat should ignore unknown items instead of returning an error

[Erik Kalkoken]

Current behavior:
If marketstat is called with a list of items and one of those items is not known to eve-central, the API returns no prices at all, but and error (http code 400 / bad request) and the following response:

A non-marketable type was given, or some other error occurred

Example (working as of 2017-JAN-19):

http://api.eve-central.com/api/marketstat/json?usesystem=30000142&typeid=603&typeid=44153

Issue:
This behavior is not ideal. If the SDE used by eve-central is not up-to-date all request will fail that contain at least one newer item.
For example I am using this API for my Slack tool EVE tools (used hundreds of Eve players), which allows to query current market prices for any item from within Slack.
As result of this behavior my tool failed to work for most ship queries (e.g. Merlin), because those contain the new Christmas SKINs which do not appear to be known by eve-central yet.

Suggested change:
The API should not return an error, but an array containing only the known items and ignore any unknown items. It should also return http code 200 for those requests