Very long atom strings can bring down slave service

I executed a clusterrunner job with an atomizer command that created very long atom strings. One was 139601 characters and it brought down the slave service with the following exception:
```
[2017-04-24 08:48:01.801] 22133 ERROR Bld1117-Sub673 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 302, in _execute_subjob
File "/home/jenkins/ClusterRunnerBuild/app/slave/subjob_executor.py", line 100, in execute_subjob
File "/home/jenkins/ClusterRunnerBuild/app/slave/subjob_executor.py", line 144, in _execute_atom_command
File "/home/jenkins/ClusterRunnerBuild/app/project_type/git.py", line 245, in execute_command_in_project
File "/home/jenkins/ClusterRunnerBuild/app/project_type/project_type.py", line 227, in execute_command_in_project
File "/home/jenkins/ClusterRunnerBuild/app/util/process_utils.py", line 79, in Popen_with_delayed_expansion
File "/usr/local/lib/python3.4/subprocess.py", line 859, in __init__
File "/usr/local/lib/python3.4/subprocess.py", line 1457, in _execute_child
OSError: [Errno 7] Argument list too long
```
According to [a stack overflow post](http://stackoverflow.com/a/29802900), the maximum string length for an argument is 131072 characters.

We should guard against any OSErrors and just fail the atom instead of taking down the slave service.