Show errors for retried jobs

When looking at the failures, the thing I am most interested in is to see the specific error which caused the failure. Clicking on each job to see its detail which includes the error is tedious and frustrating. Sometimes the job gets retried in the time between I display the Retries and click on the job, which will change the job identifier and I have to look it up again. This patch adds the Error column to the Retries table so the exception is prominently visible.

![sidekiq-errors](https://f.cloud.github.com/assets/19076/1172380/c6b74a8a-211f-11e3-9ba1-29a464506be8.png)

I have only included English localization, since I don't speak any other languages that sidekiq supports.