Pobrien/issue 408

Persistence should crash on generating an error

/cc @zendesk/zendesk-radar

### Steps to merge
 - [ ] :+1: of the team


### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-408

### Risks
 - Low: Connection to redis/sentinel does not failover. 
For now we can add two additional steps:

o log each error that comes through the handler so that I can review the frequency of error types in production

o for now, call *throw* only on a *readonly* type error (such as occurs on a failover of master)

(Vanchi and I did some testing this afternoon, and this is a reasonable approach; I'll add the code tomorrow, along with additional tests and/or modifications to the existing failover test.)