Log payload when job ends

This would have been useful for replaying events-from-token-change
failures and just seems like it might be useful generally at no cost.
I think this is a strong use case for kayvee logging! keys aren't (yet) specified, just a list of vals. But if you instead outputted as key=val it would be clearer what key each corresponds to. additionally we could act on logged errors (alert / chart / hipchat).

So if you're not tied to the current output, I'd advocate using https://github.com/Clever/kayvee-go ->

```golang
kayvee.FormatLog("gearcmd", "error", "END_WITH_ERROR", map[string]string{"function_name":conf.FunctionName, "job_id" :getJobId(job), "error_message": err.Error(), "job_data": string(job.Data())})
```
which will output a something like:
```
source=gearcmd level=error title=END_WITH_ERROR function_name=foo ...
```