* MM-10232: improve error handling from malformed slash command responses
Switch to json.Unmarshal, which doesn't obscure JSON parse failures like
json.Decode. The latter is primarily designed for streams of JSON, not
necessarily unmarshalling just a single object.
* rework HumanizedJsonError to expose Line and Character discretely
* MM-10259: pinpoint line and character where json config error occurs
* tweak HumanizeJsonError to accept err first
* ignore null array items in incoming webhooks / command responses
* consolidate code, process announcements in command response as well
* make a bit more idiomatic, add tests
* add missing file
