Give editors and admins feedback if a resource can't be parsed by the datapusher

[darwinp]

At the moment editors and admins have no way of knowing if a resource file can't be added to the datastore (e.g. an XLS that can't be parsed)

This means editors and publishers won't know if there is a problem until a user reports that the preview of that file isn't working. Note: Editors can't test manually when publishing as there is a delay between adding a resource and it being added to the datastore. It can also break later if the resource file changes and the new version of the resource can't be parsed.

Adding feedback for editors and admins would be very useful. Needs some thought to decide best format for feedback.

@shevski - Would be interested in your thoughts on this

[domoritz]

@nigelbabu and I thought about this and we plan to add some feedback about what went wrong.

[domoritz]

Done in #1183.