Status code 400 is returned if body is missing on note creation.

If a note is created with a POST request via API (`/projects/:id/notes`) status
code 400 is returned instead of 404. The resource itself exists but the request
is incomplete. Specs added to check different status codes when accessing, creating
and updating notes.

lib/api/notes.rb

@@ -43,6 +43,8 @@ module Gitlab
         if @note.save
           present @note, with: Entities::Note
         else
+          # :note is exposed as :body, but :note is set on error
+          error!(@note.errors[:note], 400) if @note.errors[:note].any?
           not_found!
         end
       end