override exit codes

[skinowski]

Docker uses: http://tldp.org/LDP/abs/html/exitcodes.html
Make sure FDK does not allow functions to exit with these codes.
For example, 137 exit code is currently flagged as OOM in fn-agent, therefore functions should not be allowed to exit with 137 themselves.

[zootalures]

The FDK should probably handle System.exit with a security manager override and trigger a meaningful error to function logs when this is called (e.g. "Function tried to exit VM ") if we did that then FDK images can only return 0,1,2