Adapt template to latest changes in harvest job error dictization

[seitenbau-govdata]

In #529 we changed the dictization of harvest errors. While this solves the problem #528 it causes a problem with displaying the error messages in the job overview.
Therefore we would fix this by using a tuple to restore the old behavior.

Details
Since the update to sqlalchemy 1.4.x the harvest error objects returned by the database in https://github.com/ckan/ckanext-harvest/blob/master/ckanext/harvest/logic/dictization.py#L79 are no longer serializable which causes commands like harvester reindex or harvester clearsource_history to fail if there are any harvest jobs with errors. In #529 these harvest error objects are transformed into a dict to fix this problem.
By doing so this data can no longer be accessed by index but this is expected by the HTML template (https://github.com/ckan/ckanext-harvest/blob/master/ckanext/harvest/templates/snippets/job_error_summary.html) and potentially by further CKAN extensions.
So our proposed solution would be to use a tuple for the data to regain the old behavior and still fix the serialization problems.

[seitenbau-govdata]

Could somebody take a quick look if this is a viable solution? Maybe @amercader or @pdelboca

[amercader]

@seitenbau-govdata I think it's very unlikely that other extensions are using this object so I'd rather update the templates to use a dict than return a tuple. Dicts are the preferred output format for the API anyway

[seitenbau-govdata]

@amercader Thanks for the feedback.
We've made a new commit with changes to the template.