Add compiler errorformat for `bats --tap` output

```viml
compiler bats
set makeprg=bats\ --tap\ %
```

Using https://github.com/justincampbell/use/blob/master/test/cli_test.bats as an example and changing the expected exit codes:

![screenshot](http://f.cl.ly/items/021Z2i0a2a2V2A3c0r0g/Screen%20Shot%202014-06-04%20at%2010.00.29%20AM.png)