Change error description sections elements to be hashes.

It is much easier for erlang to validate the json that chef-client sends up if exception_data.description.sections is of structure:

[{"str1" => "value1"}, ... ]

As opposed to current structure of:

[["str1", "value1"], ... ]

This change was achieved by updating error_description.rb. I also updated relevant test.
Merge pull request #893 from opscode/error_description_formatting

Change error description sections elements to be hashes.