Add more robust error handling to server

Changes:
- Use the proper RESTful HTTP status codes for all endpoints (e.g. 204 rather than 200 when the response body is empty)
- Add consistent and more helpful error messages
- Handle invalid route parameters by checking if jamid returns empty objects (e.g. invalid contact IDs)
- Use res.send() rather than res.json() for consistency
- Handle three new signals

GitLab: #111
Change-Id: I1d48dc4629995ab9a96bb2086a9aa91f81889598
diff --git a/server/src/routers/default-moderators-router.ts b/server/src/routers/default-moderators-router.ts
index 797aac3..06d79ca 100644
--- a/server/src/routers/default-moderators-router.ts
+++ b/server/src/routers/default-moderators-router.ts
@@ -51,7 +51,7 @@
 
 defaultModeratorsRouter.put('/:contactId', (req, res) => {
   jamid.addDefaultModerator(res.locals.accountId, req.params.contactId);
-  res.end();
+  res.sendStatus(HttpStatusCode.NoContent);
 });
 
 defaultModeratorsRouter.delete('/:contactId', (req, res) => {