Add Integration Tests for Server Error

- Clean up uncovered leaks in an error scenario
- Rename `riak_error` message to `riak_server_error` to avoid confusion
- Change how `riak_c_cunit` either runs only integration tests or unit tests

Note: This PR exercises error conditions so you will see these errors in the stderr output:
```
2014-03-23 00:50:29 EDT ERRO [27] ERR #0 - Invalid equality query undefined
2014-03-23 00:50:29 EDT CRIT [27] CALLBACK ERROR: Invalid equality query undefined
```