Fix multiple errors in resque-web failure tab

In the multiple failure backend, several methods do not have default implementations or they have implementations with incorrect method signatures.

I'm using resque-retry with the multiple failure backend, as documented here:
https://github.com/lantins/resque-retry#failure-backend

This fixes the errors I encountered with the failure tab in resque-web. I'm using this branch in production successfully.

If anyone wants to add tests for this, would be great :)