Return correct error message when given invalid json

[tleyden]

From http://www.couchbase.com/communities/q-and-a/couchbase-lite-notfound-error-when-doing-putpost-using-rest-api

JSON only allows double-quotes, not single quotes. But the response from CBL should be more informative (at a minimum it should be a 400, not 404.) - See more at: http://www.couchbase.com/communities/q-and-a/couchbase-lite-notfound-error-when-doing-putpost-using-rest-api#node-2741