Improve slurm_jobscripts:send_job error handling

[cmd-ntrf]

Two potential scenarios handling are added to the send_job function:

• Python requests raises a ConnectionError exception when posting - log API is unreachable. Previously this scenario would crash the process as the exception was never caught
• The post return codes is 503 - log API is unreachable. In this scenario, we most likely tried to reach the server through a reverse proxy and while the proxy is up, the API is down.