So my concern about expecting a proc is that the user would just end up recreating [this safe-to_replay method](https://github.com/kickstarter/slave_pools/pull/9/files#diff-adece591945532f8f3418b418ab6bde8R126). All of the logic in the proc would still have to be based on the error class and message, and I imagine it would just be based on selecting which errors to rescue (as we are currently doing).

But good point about the config maybe being a bit complicated. To make it slightly easier, I changed the error format to be all strings, instead of classes. Hopefully this makes a yml config pretty intuitive:

```
no_replay_on_master:
  Mysql2::Error:
    - 'Timeout waiting for a response'
    - 'other error message'
  TimeoutError:
    - 'this error message'
```