I get this error if I stop a dev server, ``rm -rf tmp`` and start it again:
```
`mkdir': No such file or directory @ dir_s_mkdir - /Users/angusmcleod/discourse/discourse/tmp/pids (Errno::ENOENT)
```
This fixes it.
See: f3549291a3 (diff-26ac62db6c6a4582de3bbf2615790c23R22)