Improve error message if tproxy method is used without running as root.
Improve error message if tproxy method is used without running as root.

When the tproxy method is used, sshuttle must be run as root:
https://sshuttle.readthedocs.io/en/stable/tproxy.html

Prior to this patch, sshuttle would encounter a exception and print a
message about how a setsockopt() call had a "PermissionError: [Errno 1]
Operation not permitted."

With this patch, we catch this exception, print a more understandable
error message, and exit.

The lack of error message clarity caused at least one bug report: #136

sshuttle/sshuttle,sshuttle/sshuttle