Queue API response should not include error builds

The logic to get the build queue [looks like](https://github.com/box/ClusterRunner/blob/master@{2017-07-10}/app/master/cluster_master.py#L71-L76):
```python
def active_builds(self):
"""
Returns a list of incomplete builds
:rtype: list[Build]
"""
return [build for build in self.builds() if not build.is_finished]
```
The `build.is_finished` property includes the CANCELED state but not the ERROR state. This leads to the `/queue` API returning any build that ever ran that ended in ERROR state.

I'd like to get rid of `build.is_finished` entirely and have calling logic specify exact build states.