916729 - Fixed auth failures to return JSON documents containing a
programmatic error code and added client-side exception middleware
support for displaying the proper user message based on the error.
Merge branch 'master' into jdob-916729
Changed auth decorator to use server-side exception middleware. Also
fixed handling of expired certificates in the client exception handler.
Added docs to clarify the expiration date retrieval.
Merge pull request #472 from pulp/jdob-916729

916729 - More detailed error messages for auth failures