Check for existing notes on POST and dont override them

Previously one could override notes in FreeURL-mode by sending multiple POST requests to the /new/<alias> endpoint. This commit adds a check for an already existing note with the requested alias and returns a HTTP 409 Conflict error in case that happens.

Signed-off-by: Erik Michelson <opensource@erik.michelson.eu>

lib/errors.js

@@ -20,6 +20,9 @@ module.exports = {
   errorBadRequest: function (res) {
     responseError(res, '400', 'Bad Request', 'something not right.')
   },
+  errorConflict: function (res) {
+    responseError(res, '409', 'Conflict', 'This note already exists.')
+  },
   errorTooLong: function (res) {
     responseError(res, '413', 'Payload Too Large', 'Shorten your note!')
   },