The notes API documentation updated with return codes. API now returns `400 Bad Request` if
required attributes are not present. Return codes are documented now, also tested in added tests.
The documentation now reflects the current state of the API.
Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.
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.
