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.

[rkaw92]

I would say functions should not be able to exit with any code at all. It doesn't seem useful to stop execution without producing a response. This also probably messes up Hot Functions.