Better error messages

[joetsoi]

• Change "Saving chunk 0" to "Saving records 1 - 251'
• Change error handling to use util.HttpError to store better and more granular error messages

[joetsoi]

note: these tests fail because it needs https://github.com/ckan/ckan-service-provider/pull/19

[seanh]

Closing this, going to try rewriting it a slightly different way, see https://github.com/ckan/ckan-service-provider/pull/19