BuildRunner calls cancel twice when build fails

Output from build runner:

```
Build is running. (Build id: 1)
Polling build status url: http://jharrington.local:43000/v1/build/1
0 of 4 subjobs are complete (0.0%).
1 of 4 subjobs are complete (25.0%).
Build aborted due to error: Error occurred while handling results of subjob 1.
Build aborted due to error: Error occurred while handling results of subjob 1.
Script aborted due to error!
Cancelling build 1
Executing teardown callback: >
Cancelling build 1
```

We should only call cancel once.

Also, as a side note, we shouldn't be depending on the client to stop failed builds from continuing to execute. The purpose of the client's cancel request should be just to cancel a build when there's been a client-side error.