Fix JMAP getMessages behaviour

[rouazana]

getMessages has currently two related issues:

• first defining properties like "properties": [ ${Json.stringify(properties)} ] make a json array of a json array, instead of a json array. Consequence: the JMAP request cannot be parsed, and the server errors it
• second JmapChecks.noError does not detect the error returned in this case, which is nevertheless a perfectly valid JMAP error [["error", etc.]]