It will return the counts for the 20 most common errors for that
particular job. These will available when calling harvest_job_show.
Also refactor the harvest source status object to just call
harvest_job_dictize on the 'last_job' key, as it has all the
interesting fields anyway.
To make maintenance easier and better support the upcoming auth checks,
the logic layer has been refactored to mimic the structure of the one on
CKAN core: separate actions and dictize functions and logic functions
receive a context.
Only get functions are included in this commit.