OSError brought down slave

This occurred on the slave (1028) after subjob execution. Nothing looked weird about the subjob directory itself. Maybe this would succeed with a retry?

Either way it should not bring down the slave.

```
[2015-07-17 01:35:33.568] 6327 ERROR Bld611-Sub1273 unhandled_excep Unhandled exception handler caught exception.
Traceback (most recent call last):
File "/home/jenkins/ClusterRunnerBuild/app/util/safe_thread.py", line 18, in run
File "/usr/local/lib/python3.4/threading.py", line 868, in run
File "/home/jenkins/ClusterRunnerBuild/app/slave/cluster_slave.py", line 296, in _execute_subjob
File "/home/jenkins/ClusterRunnerBuild/app/slave/subjob_executor.py", line 113, in execute_subjob
File "/home/jenkins/ClusterRunnerBuild/app/util/fs.py", line 118, in compress_directories
File "/home/jenkins/.virtualenvs/clusterrunner/lib/python3.4/tarfile.py", line 1930, in add
File "/home/jenkins/.virtualenvs/clusterrunner/lib/python3.4/tarfile.py", line 1923, in add
File "/home/jenkins/.virtualenvs/clusterrunner/lib/python3.4/tarfile.py", line 1952, in addfile
File "/home/jenkins/.virtualenvs/clusterrunner/lib/python3.4/tarfile.py", line 241, in copyfileobj
OSError: end of file reached
```