* Improve error messages related to sshuttle server.

There are many GitHub issues related to the cryptic message:
fatal: expected server init string 'SSHUTTLE0001'; got b''

The code that prints that message is after another check that is
intended to verify that the server is still running. This code was
faulty since the server is still running when rv==None (but exited
when rv==0).

I corrected this problem and then investigated ways to clarify the
error message. I added additional exit codes for the server: 97 (exec
in the shell returned), 98 (the python exec() function called
returned). The end result is that the cryptic error message above will
now print a more appropriate error message that should aid in
debugging.

I also changed the server so that it catches Fatal() and exits with
exit code 99 (like the client does). Previously, it was just an
unhandled exception on the server.

I suspect some of the error messages were caused by restricted shells.
I also investigated and added comments about how sshuttle might behave
if it is being run on a server that has a restricted shell.

This commit also replaces a couple of exit() calls in cmdline.py with
'return' since exit() is intended for interactive use. This change
doesn't impact the server.

* Remind user to exclude remote host when server exits with 255.