Add a function for returning the appropriate "no access" API error code.
Review Request #5671 — Created April 2, 2014 and submitted
We had several places in WebAPIResource that calculated the proper error
code to use for indicating that access isn't allowed for a resource.
Consumers then had to repeat that logic, which was silly and pointless.
This introduces a function, get_no_access_error(), that performs the
Unit tests in Djblets and RB pass.