Display error if the app doesn't start

[MattiSG]

If I already have a process running on the port set up for the app (default 8000), the app won't start as it cannot bind to that port.

However, Ansible will output:

TASK [openfisca_api_fr : Enable and start the systemd service] *****************
changed: [openfisca_api_fr]

TASK [openfisca_api_fr : Check that OpenFisca Web API is actually started] *****
ok: [openfisca_api_fr]

The systemctl log will show that the service is inactive (dead).

This error should be shown in the Ansible logs.