Gunicorn Socket file is missing
Try running the script without doing exec ~/.virtualenvs/hello_project/bin/gunicorn.
Instead try running it with gunicorn then the args.
This is assuming you have permission to install gunicorn on the system without a virtualenv.
I have a gist that starts the same process you're trying to run. (Django, Nginx, Gunicorn)https://gist.github.com/marcusshepp/129c822e2065e20122d8
I see you're using supervisor so you're going to not want to use the --deamon option that I use in my script.